While all of these ideas make sense, the one thing that sticks out for me is -- who else could he be? Seriously. He's obviously one of the Creators, but the only one that's "loose" is Fen'Harel, and he knows so much about him, to boot.
And he's obviously taken on another face when he becomes Solas, since the one he was wearing was "killed" in the Fade. (No, I don't really know what that means, but the word "killed" is in the sentence, so I'm taking that as fact.)
Body-surfing or taking on a meat suit disguise? Or something entirely different? There's a billion ways they could explain this, but I just don't see Felassan being anybody else.
Again, even if he got around the physical appearance, and the voice, there's still the fact that he should have some remaining affection for Briala, if he were the same person. He doesn't. What he says about her if you support her, or end up exiling her, seems rather distant and certainly doesn't seem to hint at any prior friendship or emotional attachment. Could he be hiding it? Sure... but I think he'd tip us off somehow, if he were. Having read the book, you can't tell me that he didn't care about Briala.
Being killed in the Fade generally makes an individual Tranquil--and I think it's irreversible, in that case. Though, if the elven gods were spirits or spirit-like, I don't know, maybe it is possible?
Felassan was likely someone's slave--vallaslin, remember?--or willing servant. Obviously, to one of the gods. There are other gods in the elven pantheon, and, he communicated with whomever he was working for within the Fade. Some think it was Fen'harel who killed him, and that's also entirely possible, although for a variety of reasons it doesn't seem to be the case (in my opinion).
As for who Felassan is... beyond "probably an ancient elf who sympathizes with and/or follows Fen'harel," I don't know. We may never know, because he is now dead/Tranquil and it seems we'll never meet him, or at least never meet him in the state he was in prior to that event. (Which is sad, because he was interesting.) Some speculate he was one of Mythal's sentinel elves. I suspect he's from another group of ancient elves, and perhaps whoever he serves (I don't think that it is Flemeth/Mythal, OR Fen'harel) is awake and trying to escape the Fade. But, he could be a servant of Mythal (those creepy whispers!) and working against (!!!!) Fen'harel (which would be interesting as they APPEAR to be allies, or to have been such in the past, and he clearly admires Fen'harel). Or, he could have served Fen'harel--which would mean he's acting on his own, since I would think if Solas had any of the People stowed away somewhere serving him, he would probably not be wandering around the rest of the world.