True, Corypheus didn't seem to be in complete control of his plans... but given that Solas is the God of Spectacular Backfires, that doesn't paint the Dread Wolf in a better light. Let's not forget that current Thedas is Solas' version of the bad future, and Solas is even in less control than Corypheus was in his.
Well, no, not really. Solas's plan might have been a more or less spectacular backfire, but it was also a last-ditch effort before - from what we know - something even more spectacularly disastrous was about to happen. He saved the world and - no matter how 'bad' that world is now - he gave it another chance, for more than a few millenia, while Cory's epic ruling ruined it for everyone in about a year.
Then there's the fact that it's not the first Cory's disastrous mistake, where he - again - toyed with powers he didn't understand. After all, he was the one who took his buddies on a trip that turned Golden City into Black one.
Solas might have either not fully known - or known, but couldn't really do much - the extents of how the Veil could impact and change the world, but from the look of it he's known exactly who he was going against and possibly what they could potentially unleash on the world.
I have to disagree in that regard too. Certainly, Solas knows a lot about past Elvhenan, the Evanuris, the Veil, the spirits and the Fade than practically every other person we've met so far. BUT his knowledge starts faltering precisely when Old Gods, Archdemons, Red Lyrium and especially the Blight are involved (he also doesn't provide much insight on Titans). He questions a lot, which is fitting of his character, but never provides any answer related to those subjects while he's more than happy to show off in the other cases.
Hang on - how do you know his knowledge begins faltering?
We do know that there/s a connection between Old Gods/Archdemons and Evanuris, since they were a focus of Flemeth since DAO and in OGB world-state we get to see that not only Uthremiel uses the same kind of blue power Flemythal/Solas uses, but she explicitly calls him 'a piece of someone long thought lost' and later takes his soul into her. And we do know that Mythal and Solas worked (if they're still not working) together and known each other for a very long time.
Solas also reacts to Wardens' plans with horror, calling the whole thing a terrible mistake that could make tings 'even worse' - and it's one of the few instances in the game where he shows a lot of emotion. He's freaking out in fact - it seems that the Blight terrifies him absolutely. At one point he even calls it "the real problem", says it's not a power 'one smugly outsmarts', tells Vivienne that it's something that corrupts everything and only a fool uses it and lets us know that 'the fools who first unleashed the Blight on the world thought they're unlocking the ultimate power'... which realistically couldn't have been Corypheus and his team, because they didn't really go to the Golden City for explicit purpose of unleashing anything.
(We also have the old elven tales that mention Andruil getting mad and 'howling things that should be forgotten'' and hunting even gods, until Mythal stopped her. There are also Cole's cryptic comments in which he mentions that Solas "broke the dreams to stop old dreams from waking".)
I think it's pretty safe to assume that he knows more about the Blight than he lets out. The fact that he doesn't speak much about it doesn't mean that he has no answers - only that for one reason or another he doesn't want anyone to know.
And he's not the only one to do that. Both he and Flemythal do sound sometimes like there are mysteries about the old world they want to take with themselves to their grave (btw. when Inky asks her if she knows what they're fighting with, she she only says "more than you could ever imagine".... In fact, how is it that it's HER Well that provides an important insight into the workings of red blight magic Corypheus uses?).
...As for Titans:
Yes, it's Solas. It's his mural - that we can find in an ancient mine riddled with *his statues* and containing deep lyrium well surrounded by what seem to be ancient elves in uthenera - and it's him depicted on this mural, apparently striking the Titan.
So yes, he probably knows a bit about them too - or at the very least what happened to them and/or dwarves.
He consistently keeps pestering Varric with questions like "does Orzammar researches lyrium and did they find anything interesting?" or "Varric, do you miss your special connection with the Stone?" or makes cryptic comments like "the dwarves are a severed arm of a once-mighty hero..."
As for Descent - his first dialogue in DLC (when they're taking the lift down to the mines) is literally suggesting that there's something intriguing dwarves are connected to. He doesn't have more banters than other companions, but he's also the one who is consistently pushing to investigate the whole thing, as if he wants us to go and find out.