Something you both forget is that Mythal died, or was killed yet part of what she is remains. Remember what Sola said about his 'wisdom spirit' friend, that they are not truely dead.
I think whatever the Elvhen Pantheon is, they are beyond even what spirits are. Mythal isnt strong enough to survive another death, yet Solas is. Given his mistakes in the past, his ones lately he'd not risk Mythal at all, so we've not seen last of him, though I think the price he paid for his mistake with the Orb is that he can take a body, nor form himself. What he truely is hasnt passed nor who he is either, just reduced to a 'whisp' of what he once was, and like Mythal will take time to return or even join with another to semi return.
While everyone's reasoning for why they think Mythal took over Solas' body, rather than Solas taking Mythal's power/spirit/whatever, is solid enough, I still don't buy it. Why would Solas walk over, looking all mopey, saying he SHOULD be the one to pay the price BUT... and then sacrifice himself? The people NEED him, after all. It doesn't make sense for him to justify his reasoning for why he can't pay the price, and then go ahead and pay it anyway.
Abelas already said the dread wolf had nothing to do with betrayal. And it's already pointed out that it's Dirthamen. This is assumed to be what Dirthamen looks like:

That, uh, looks a lot like a Fear demon, doesn't it?
Weren't there wolf statues in the Lost Temple of Dirthshmirth?
They're everywhere. It drives me crazy. 