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.
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!
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!
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.