I see it a bit differently.
I honestly think it could go either way. We clearly don't have enough information on the Elvhen Pantheon to make a decisive distinction between Flemythal's situation as compared to Fen'Harel's/SolaFen's situation.
It is true that she claimed to have been murdered (this was also confirmed by Abelas) and was said to have held on to a piece of her essence and powers while searching for a "physical" host. Given these comments however, it is clear that these "Gods" are not simply of the Elvhen people, or rather they have transcended the aspect of what they were and now cannot be compared to whatever that was.
The way Flemeth describes how it feels to be "bonded" to Mythal, the discrepancy between them being "separate" beings is a slight one. They share everything, so I don't see this comment:
"The experiences he speaks about are his experiences. They weren't gained second hand."
necessarily being the best way of putting it.
I don't think it's as simple as living through a god but rather being one with a god, sharing everything, where one's feelings is as the other's. The process for Mythal's "possession" is stated to be based on choice. This would make it completely different from the possession we are familiar with a.k.a. demons. Demons possess from the Fade and control Bodies from The Fade. Their essence cannot cross over to become the entity they possess, unlike the way Flemythal describes her own situation, where Mythal's "essence" wandered and eventually "bonded" with Flemeth. They simply see through the host's eyes and manipulate their body like a puppet master, keeping the spirit of the host hostage within The Fade.
I also consider Kieran's comments about the Elvhen people i.e. why they "want to look like that" and that their blood is "very old".
This would seem to imply that even ancient Elvhen people had the capacity to shapeshift, further reinforced by the Codex entry: Ancient Elven Writing
This then has me wonder about the Pantheon's nature and abilities. It also brings into question the reason this was lost to time . I believe it has to do with the creation of the veil as well as the imprisonment of the Pantheon, much like the loss of immortality and the "appearance" of humans in Thedas.
As for Geldauran's claim
exert from Geldauran's claim: "There are no gods. There is only the subject and the object, the actor and the acted upon. Those with will to earn dominance over others gain title not by nature but by deed"
I don't see this necessarily pertaining to the Pantheon's nature, but rather giving us insight into The Forgotten Ones' perception of them and their disdain for what they represent. I don't see them as freedom fighters, but anyways I'm not going to delve into that as I already have a thread that goes in detail about my theories. What I will say is that the Pantheon could be a type of spirit/Elvhen hybrid and "The Forgotten Ones" a type of Elvhen/demon hybrid. They could all be powerful Elvhen mages or the pantheon could be of a different nature while "The Forgotten Ones" altered their own nature through their deeds to match the Pantheon. No matter what we believe,
Saying this:
"If every elf was doing this it dimishes how completely awesome Mythal was for being able to hang on like this."
is implying that Fen'Harel was like every other ancient Elvhen at the height of Elvhenan and that you know what the Pantheon's aspect and "true" nature actually was.
So far the information we have seems to imply that Fen'Harel was their (the pantheon's) kin and regarded as a brother. Which would mean he is "of" them i.e. his nature, while "The Forgotten Ones" see his attitude and rather than judging him by his "nature" they accept him through his "deeds" i.e. cunning ways.
Being of the god's does not mean he is like them. I think he saw both sides of the coin and realized those who truly suffered were the Elvhen people, thus he decided to rebel against those who would ignore the "little people" as Sera would so simply put it. The warring lead to casualties and he knew that eliminating one side wasn't enough, the shift of power would destroy the people, he had to stop the warring but also had to deal with both sides as they were keeping each other in check.
Back on track, I feel like Mythal's strength of character and uniqueness is quite intact despite Fen'Harel's disposition.
To put it like Geldauran: Those with will to survive gain title not by nature but by deed".
It's not about how she survived i.e. thanks to her nature, but why she survived i.e. her deeds. Her will is what I see as special, not her nature.
As for Fen'Harel and Solas. Again Solas could have been a dreamer who encountered or even woke Fen'Harel. They could have developed a kinship which led to their "union".
I'll finish with these, copy pasted from my theories thread:
This leads me to my next topic... SolaFen. First of all, let me start by explaining what I think of his origins. I believe Solas to be a willing "vessel", much like Flemeth. I also think the stories he's told us (the inquisitor) and his humble origins could be the truth, just not the whole truth.
Lelianna: "But he did give the name of the village where he grew up, noting that it was small, unlikely to appear on any map."
Lelianna: "They recently located the village... or what remains of it. It is a ruin, as it has been for centuries, its name preserved only in degraded form in Ancient Tevinter mysteries"
I believe Solas (or whatever his name was at the time) encountered Fen'Harel during his time exploring The Fade. He must have been in agreement with Fen'Harel and gave his body willingly. Given the conversation between SolaFen and FleMythal (When SolaFen explains that he didn't have enough power to unlock the Elvhen orb given his long slumber and recent awakening), I'm guessing the village he said he was born in was simply misdirection and that Solas is not in fact centuries old. I also say this because Mythal's vessel (Flemeth) is growing quite old physically and has already lived centuries. She needs willing replacements in order to sustain herself. If centuries mean nothing to Fen'Harel and Mythal however, than a long slumber might be millenia and being awake for a few centuries might be a recent awakening. I might be stretching it, but even if that was the case then SolaFen should also look as old as FleMythal does. Since that's not the case, I'll assume Solas was lying about his birth place and he bonded with Fen'Harel recently. If I had to guess, I'd argue that the location was in fact a place he visited while in the Fade, which is probably where he found its name.
and this:
That could be the case, but then why didn't Abelas recognize Fen'Harel while they were speaking, If Fen'Harel was a figurehead during the rebellion. Clearly Abelas was around during the time of the Elvhen Pantheon given his statements about Mythal and her "murder". For that to be explained, then no matter what, the physical body (Solas) would've had to be altered somehow. Either Fen'Harel can shapeshift into another humanoid being (unseen so far) or he can create illusions of some kind. Then there's also the why and how Fen'Harel "woke up". Also, we don't really know what the Elvhen Pantheon really were, the same way we know nothing about "The Forgotten Ones" (though Geldauran's claim could imply they were of the Elvhen people). They could have been similar to spirits (Elvhen Pantheon) and demons (The Forgotten Ones) who become something similar to Cole. Anyways a lot of ifs in here, so I'll stick to SolaFen for the time being.
Anyways just something to think about.
Oh and sorry for the very long response. I just like to be thorough.