I sometimes wonder if Solas was originally supposed to act more like you'd expect Fen'Harel to act. I keep thinking of those concept art pictures of him with hair where he's all devious looking and smirking and then that one piece with everyone shouting at the war table and him in the background smirking like he's amused.
If so, I'm glad they didn't go with that. He has a few peeks of wry humor and slyness every now and then in game but they don't beat you over the head with it. He very effectively gives off the vibe of a once confident, arrogant, cocky and hot-headed guy that grew up a lot after struggling for so long and failing so badly. That's 1000x more interesting than someone who acts more like a typical trickster character.
100% Agreed.
Although, when Solas talks about his younger self and how he was "cocky, hot-blooded, always eager for a fight" and "a much younger elf who was sure he knew everything," I imagine that's what he was like. I've pretty much decided that his earlier concept art was him when he was in his prime way back in ancient Arlathan. Before he grew up and became the wise, cerebral, hot-blooded egg-head we know and love today. 
I assume he probably slept after trapping the elven gods. Either because he was completely power-drained, or he might have been compelled to sleep as a form of revenge.
So he saw the initial fall, attempted to prevent it, but probably missed out on Tevinter coming in and sacking the place after the war.
To be honest, I think he went to sleep to hide from his guilt.
While I'm sure Fen'Harel was cocky and arrogant, it sounds like his decision to lock them away was not one he made lightly. Or, if it was, it's possible he had believed or had convinced himself at the time that it was necessary, that it was for the greater good, whatever. However, he probably knew on some level that it was was wrong and avoided facing the consequences. Went deep into uthenera, away from this world, away from the mess he had created, probably telling himself the People would be better off and/or it would sort itself out eventually. But it didn't sort itself out. It got worse and worse, and by the time he finally emerged from his slumber, the People were so far gone. I think the realization would have devastated and humbled him greatly, if he wasn't already matured or maturing from his deep slumber and/or other important discoveries, and now he wants to fix his mistake.
The reason I think this is some party banter if the Inquisitor learns who Blackwall is, then forgives him and brings him back into the fold.
- Blackwall: You haven't said much to me since...well, you know.
- Solas: There is little to say. I assumed we were alike. We'd seen war, knew its terrible costs, but understood that it was necessary. But there was nothing necessary in what you did. You did not survive death and destruction. You sowed them. To feed your own desires.
- Blackwall: I know that. I see it every time I look in a mirror. I try to make up for it.
- Solas: By wearing another skin. You ran away rather than face what you had done. You wasted your time.
It sounds a lot like what Solas is pushing his own misplaced guilt onto Blackwall.