Alright... here we go!
1. We have no evidence or proof that Mythal was human other than three clues: the myth of her origin, the visual used in the Well of Sorrows, and the race of each of the women she has inhabited so far. Mythal comes from across the Ocean - the same way humans came to Thedas. In the myth, this is presented to us as her rising from the ocean as compared to traveling there by ship. But, given Solas' opinions on the nature of Godhood, the chances of Mythal rising from the sea Aphrodite style is highly unlikely. What is likely, however, is that her origin was dramatically oversimplified throughout the ages, the truth turning from myth, to legend, and her.. humanity, if you will, was lost in the process, and she became even more ethereal in her conception.
The visual in the Well of Sorrows is fascinating - I don't know why she has human ears, even if it were an overlap. I don't have an answer for that. We could believe Bioware made a mistake using the wrong body model in the scene. I just find that unlikely given the importance of the Well of Sorrows sequence in the narrative. That would have been poured over by the art, animation, and cinematics team for weeks. I believe it was a conscious decision and I don't think Andraste is the answer. Given that Mythal's slaves/ followers/ chosen were Elves, the majority of entities likely drinking from the well and contributing to its collective consciousness was also elves... I find it strange that one woman would overshadow the rest.
Last point in this section is that all of the hosts for Mythal to this point have been both human AND female. All of her daughters, in all of her lives beginning with Andraste and beyond, all female. All human. Interesting to note that Morrigan, one of her daughters, breaks that line of only-girls when she has Kieran, a boy, who holds the soul of an OG. It's a break in the pattern and because of that, important.
*snip*
Excellent responses so far. I'm going to start replying point by point (abandon all hope, ye who enter here! No really, I will try and be succinct, I can do it!):
1. Mythal as a human. I would still say that these amount to pretty circumstantial evidence; the oversimplification seems plausible to me, this runs along the lines of the Protheans evolving into Asari goddesses over time. But the connection of humans coming across the sea and Mythal being ostensibly one of them? All I've read and heard about the interaction of Elvhens and the first humans indicated only a kind of cordial curiosity - why under the sun would they raise a shem to godesshood? If she came first, a forerunner of humans to come, would she not have brought the quickening? It seems to me that all the evidence of the first meeting between Elvhen and humans was a powerful, dominant race (Elvhen) already with an intact pantheon that helped and hindered them as gods do, and besides, would they not have reacted as to gods when they first saw the likeness of one of their goddesses? At the time of their first meeting, according to the lore, their gods were still there at the time and it was only later that they were sealed away, so they would have had some way to recognize her kith and kin.
The visual I interpreted more to be a clue for us to realize the truth of Asha'bellanar being Mythal right now, although I'll grant that there isn't enough evidence either way to carve it in stone. The fact that she inhabits human bodies... I interpreted that to be more that a single person has a keen sense of justice and is in a position to change the course of events in the world - which Andraste clearly was based on what we know, and that Flemeth is and was, considering her power and what she has done. If this is true, then I would need to substantiate why she didn't choose justice-oriented elves as hosts.
I'm not sure I can do that, but: first, elves are no longer in a position to change very much. The power has to start from a human, otherwise it will just be another elven uprising to squish ruthlessly. Second, as Solas (ostensibly one of those gods) is always so keen on distinguishing himself as an individual first, elf a far less important second. So why should Mythal be any different? She sees the power of one particular human, not caring about her race, and through one human realizes that it's a comfortable "skin" for her designs (and a convenient one). Third, the willpower. What elf would not be completely overwhelmed by being touched by their goddess? I'm not sure it wouldn't end in an Anders/Justice disaster of lack of balance and emotions feeding off one another. The choice could have been that simple, which eventually became a preference.
An interesting point to this: what if Andraste was truly an elf? As history is known to distort everything, who is to say that she wasn't actually an elf, a fact that was kind of conveniently forgotten? Her relationship to the elves became something different (why would Andraste have cared so much?) True, I can refute this point of mine with the visions our Wardens saw in the Temple of Sacred Ashes. Shartan says that the elves rose to help her and fought beside her despite her being a human that sang the praise of the shem's Maker. Yet, we know how flimsy such visions can be, and history is always written by the victors and always with the bias of a given group (as we clearly see throughout DAI codex entries). Idle speculation only.
Additionally: that entry about Andraste's line of female descendants is oddly reminiscent to the legend of Flemeth and her daughters. This would, to me, prove a plausible connection between Andraste and Flemeth at least, even if not the connection of Mythal.
For a final point: I concede that it is odd that Mythal (if that's who she truly was) is represented as a human when all the minds poured into the Well were ostensibly elves. I would still say that it's an indication of elves of a later time possibly encountering Mythal... without knowing that it was really her, and they only realized the truth on a subconscious level. This is a Well of wisdom, too, remember - part of wisdom could be that Mythal is alive and no longer elven. But I must point out that Abelas is convinced that Mythal was murdered - i.e. she cannot be alive. Where would she hide? Where no one would expect her, of course. If someone wanted to kill me and thought me dead, I would not return in any way sense or form to the world anywhere where I would be recognized. I.e. Asha'bellanar is interested in elves but will not interact with them despite her being their goddess. (Not that they would necessarily believe it.) Andraste also managed to help the elven uprising in Tevinter, without directly seeking them out and championing them. (Would she have had a human army at her back with all that entailed if she had been an elf?)
2. I believe Solas is telling us the truth. If we look at the themes of DA:I a HUGE part of it was the tendency of common people to deify and elevate those they see with power they do not understand. It happens with the Herald/ Inquisitor no matter how hard you deny it with your own mouth. Everyone, literally EVERYONE (with the exception of Solas), is willing to see you as something more until the sequence in the Fade. Even then, people choose to reject the truth. When analyzing myths and tales of origin within religious narratives, more often than not, they were constructed by the people to explain things they couldn't understand. A person walking physically through the fade? A woman guiding her to safety? Gotta be the spirit of Andraste. This person was sent here to save us! The inquisitor is an elf... welllll... I suppose that shows how far the Herald has risen! YEAH, racism being used to explain how divine you are!
2. Solas and godhood. I concede that this godhood question is blatantly called into question just by all the things we see and hear as the Herald of Andraste (as a Dalish elf, come on! And I can't imagine this being any prettier with a Qunari Inquisitor). However, I would still say that we only get Fen'Harel's point of view, which is as limited by his beliefs as anyone's is concerning this kind of iffy spiritual question. True, I'm not going to outright and say that he's a god end of story - I will say that he is not necessarily anything as "simple" as an ancient elf with superior magical knowledge. That makes no sense to me - my impression was that magic was an intrinsically wide-spread trait of the Elvhen in general. So what would make this particular magic-wielder so special? Orbs of power...? Well, I could say that any technology is as good as in everyone's hands almost from the moment of creation, but most especially after its first use. It takes time, yes. It may be considered as a distinctive quality in the first to use it. However, for godhood, the Elvhen would have had to be pretty primitive compared to this "group" of elves who gained so much power as to become gods in the eyes of the people. And Solas is and elf, and we've seen other "gods" inhabit the bodies of humans - all this mixing and mashing says they are neither elves, nor humans - they are something beyond that. If they are not gods, they still could be particularly potent spirits, the original spirits that all other similar spirits come from. Consider their superior powers (I'm thinking of Flemeth here most, she's the most blatant in her displays of power).
Having said that, I do like the idea of Solas being just an ancient, immortal elf - yet to have power that surpassed his kin to the extent that he single-handedly sealed away a bunch of beings that were considered gods and their enemies? It might indicate that while the others were gods, Fen'Harel could have been something more akin to Corypheus - an ancient elf who braved the home of the gods themselves and outwitted them. I know this is getting far-fetched, and I have to add to this the sorrow he feels for them, that does not sound like the reaction of someone who did what he did out of a desire to preserve them, his "equals" - but there is that bit about him being the formless one. He doesn't quite fit in anywhere, does he?
I don't know what Solas' plans are. All we know is that he wishes to rectify a mistake of his past, a mistake so scarring to who he is as an individual, he cannot abandon his efforts to fix it. Cole's party banter with Solas post-break up where he's trying to help Lavellan's and Solas' emotional pain tells us the following: 1) Back when things sang the same, Solas felt an emotional pain - either due to the situation of the elves at that time or due to his actions at the time. 2) Clearly something about the Inquisitor, by being real*, could change everything (even Solas admits this in the Haven scene - he feels the whole world change). 3) "They sleep, masked in a mirror, hiding, hurting, and to wake them... (Gasps.) Where did it go?" - those who are sealed behind the Eluvians are in uthenera. They're clearly hiding from something they wish to escape, but in doing so they're experiencing pain. The only way to solve THAT pain, which Solas is quick to tell Cole he can't fix, is to awaken them with some sort of MacGuffin we don't have enough information to name or place.
Another Cole-Solas banter hints that Solas envies Cole because Cole can be happy because he can forget people and events and move on without changing his nature. Cole offers Solas two ways to heal the pain of his memory: 1) Remember them. In remembering them, cherishing them as they remain in his memory. 2) Let them go. This could mean "forgetting" them, but I think it has more to do with moving on. Accepting that they once were, but time requires that his mind move down different paths. It's clear that Solas appreciates these offers of help, but refuses to take either path. He remembers them both as they were and as they now are - at one point great, and now in pain. He can't let them go because of his sense of responsibility for the situation, so he can't move on. These are emotional choices Solas himself has made and rejects Cole's solutions. Cole reminds Solas that he souldn't feel guilty because it wasn't about Solas being right or exerting dominance over them, it was about saving them, which was right.
So TL;DR Solas is consumed by guilt at whatever action he did because he is unwilling to relinquish the memories he has of those he sealed, even though the course of action he took was right and it did save them. Knowing what we know about Solas about free will, maybe he feels guilty about what he did because it robbed them of their choice - even if it was self-destruction. Yes, I think he feels shitty about how things went down for the elves as time progressed - I'm sure he had hoped they would have escaped slavery, ignorance, poverty, and servitude. He knows he can't rewind time (dear god please don't let there be anymore time magic in DA.. it's such a mess and never satisfying), but perhaps by bringing these Gods back, the People can regain what was lost. If these Gods self-destruct, it's by their own choice, the very thing Solas feels he robbed from them.
While Solas idolizes a time when the Fade and the Physical World were one, I am unsure if he actually lived it or has only experienced it via memory in the Fade. We have no way of knowing until more information is given. I just question if that's his actual goal because his conversations with Cole seem to point at something else. The rejoining of the Fade to the Real World, however, might be the fall out of whatever it is he is planning to do, just not the end goal in and of itself.
I'll continue the rest in another post or else this will be a terrifying wall of text no one will ever read!
The Eluvians are a curious question for me. I take it that the gods are sealed there - could it not be that basically the gods are not dead, not corrupted Old Gods who were destroyed in the blight (though that wouldn't explain Solas' disdain for Gray Wardens), but actually are intact inside the Eluvians? Maybe even in the corrupted ones... or there is a permanent connection open there. Hmm, I have to think more about this.
As to the rest - Solas' motivations seem pretty clear to me throughout and I agree with all you say; however, there is that last moment when Mythal "transfers" to him, or transfers something to him at any rate. Or Fen'Harel takes something in a moment of weakness. That look... he wasn't smiling, but something happened there that I think was way more important than it's being given credit for. With beings as powerful as Mythal and Fen'Harel, their goodness or evilness (in my opinion) becomes an utterly obsolete question. Even benign forces can be viewed as evil for their sheer force of presence, and the concept that their actions go beyond small goods and ills in the everyday perspective of a mortal. That said, the power play between them is obvious - it seemed to me like the kind of "we've been enemies so long that somewhere along the line we turned into allies," which fits the lore we know of Fen'Harel. I'm not entirely convinced that what Solas considers as good will be appreciated as good from, say, the Inquisitor's perspective... or anyone else alive, either. I see that he is consumed by guilt and pain over what happened, he clearly messed up and he clearly regrets - I don't think those expressions were fake.
Yet... that look. Can I truly trust that his designs for Mythal and the other gods are what they seem to be?
I agree about the Fade/"real world" thing being potentially a kind of byproduct and not the end goal. He's clearly focused on the gods, what he truly did or did not do as Fen'Harel, and those concerns go beyond a mere description of the world in a certain state. What he says also points to my earlier question, though, who could be considered a god in a world (if we posit that Solas was speaking of an actual experience from the past that existed before the fall of the Elvhen) where such magic is so intrinsically interwoven with everything real?
EDIT: *REAL: What does that word even MEAN!? Sometimes I feel that "real" means spiritual to Cole because when he talks to a mage Lavellan he mentions her pulling stuff from the fade and in the process making the physical world more "real", but at others I feel like "real" refers to those that are both spiritual and physical - like himself, Solas, Cassandra, and the Inquisitor. They all (by their nature as spirit-human hybrids or power) exist in some way in both places.
That is a subject for a lengthy philosophical debate, my friend. Even within just the scope of DAI alone, what is "real" is such a subjective question to everyone involved in this divine mess. However, I posit that Solas, Cassandra (after her reveal of how she became a Seeker) and the Inquisitor (who shines too brightly) - this I can believe that basically they are real from Cole's subjective perspective, i.e. some connection to the Fade. Awwww Solas, why didn't you LISTEN to Cole?!
OK, I see I have utterly failed at being succinct. I hope that at least some people will humor me with their attention, though I rob it.
I look forward to the rest!