There was an error with Solas' dialogue at the point where you have to decide whether to drink from the Well. He's supposed to beg you not to drink from it yourself but he doesn't, it's missing. Patrick Weekes confirmed that it's only flavor that's missing though, no plot-important lines.
Really? This exchange left me totally bewildered in my first playthrough as a human who had gained his respect. I thought to try the other option this time around, but because that mysterious dialogue isn't in there, and its still going to sound like Solas is asking me to do it instead, and I don't see how my hopelessly-in-love-with-him Lavellan could realistically do anything else. Flavor is important.
I wonder if they will ever patch it in. I wonder if it's actually in the files somewhere and just wasn't triggered correctly, or if it was a total oversight and never even recorded. Doom on it. And doom on his bugged out personal quest too while I'm at it.
More on topic: I'll happily back your theory way back on post #3. Thank you for sharing it, and putting so many of the pieces together. Such feels I have for this elf after beating the game. It's nice to have a solid theory to play with in my head.
As for the post credit scene: I think Flemeth simply gave the spirit of Mythal plus one to Solas. Maybe in the instant that he knew her intent, he saw it as the punishment he thought he deserved, and became a willing vessel. Maybe the reason Flemeth apologized was because she knew his shoulders were already bowed with the weight of the mistakes he had made and centuries of regret, and now she's going to give him all of Mythal's and the unknown one as well. Meanwhile, she can finally die, or pass out or whatever without their probably overbearing presence, and go back to being whatever she is without them.
I am convinced that Solas did nothing malicious there, and he walked away with so much more regret, angst and sorrow to bear from the new tenants in his immortal body.
One tiny little thing I do wonder about the post campaign: Since Cole seems to broadcast the hurts he hears in real time, and they always seem to be from someone nearby, does that mean Solas was present at Skyhold when we overheard it?