Well. It may have been a poor decision on his part, what with the whole elf god thing and not telling anyone, but it doesn't bother me since once you do know you have the option of breaking up with him or continuing the romance. Which means that however you shake it, Lavellan can now make a wholehearted decision depending on who she is as a character and what she wants. Destruction of the known world notwithstanding.
I think it's the key moment where you really know if you can accept the whole person or care only for the part you did know.
In some ways, the Trespasser reveal is only the beginning of getting to know who Solas is, from Lavellan's perspective at least. Even for the player, you're forced to ask yourself how much of the Solas you knew was real. I ended up concluding that a lot was - but even so, finding out his plans does change a lot of what you knew of him before. It's not just a case of adding 'more' to his character.
There's knowing acceptance, there's blind acceptance, and there's taking a leap of faith. In my mind, choosing 'redeem' is the leap of faith. It's counting on there being enough good in Solas that he's worth trying to save, whether you succeed or not. That's my Lavellan's choice. Even so, she's effectively given up on the romance (though not the love) for the moment because it matters much more to her to save the idea of who she thought he was. Post-Trespasser, she would have to be questioning her own judgement a lot. I remember during my main game playthrough - pre-reveal - thinking to myself that I hoped Solas wouldn't turn out to be 'evil'... it was just a passing thought. I didn't actually think he'd become a villain, but there's a reason I had the thought in the first place. Sometimes he's just a little too mysterious and reacts to things in an odd way. I headcanon that my Lavellan was often frustrated and confused by his behaviour but saw enough that was definitely good and lovely about him that she wanted to take a chance and keep getting to know him.
We're still being asked to take that chance, in my opinion. We don't know everything about Solas yet. My Lavellan is having to wrestle with the fact that if she's honest, she may have been really wrong about fundamental aspects of his character. It would never have occured to her that he saw her and their friends as anything less than people. His behaviour suggested the exact opposite - that he saw everyone as an individual of fundamental worth. She never thought he'd be willing to destroy a world - his anger at the Wardens, his disdain for Morrigan's eagerness to drink the Well, not to mention the fact that he's helping you against Corypheus, all indicated that he found 'at all costs' types arrogant and contemptible. She never thought of him as the kind of person who would decide that he knew what was best for everyone - see his deep antipathy to the Qun. And she believed that his anger at the Dalish actually meant that he cared deeply about the plight of elves. (And he does, just not necessarily modern elves.) All these things formed her basic understanding of the person she fell for. All are being called into serious question.
It's on a different level than, say, Blackwall, or Zevran's past with Rinna. I romanced Zevran, and never really had trouble accepting with his past, because it didn't shake my view of him as a fundamentally good person - if anything, learning about his regret confirmed it for me. Same with Blackwall.
We may not know the whole story with Solas yet, and I still believe he's really good at heart. But he may also have a flaw that makes it impossible to co-exist with him. My Lavellan may have to accept that, in order to be true to herself. And I may have to accept that about Solas in order to do justice to his character. 
I'm sorry if all this comes off as terribly pessimistic. I still count myself on Team Optimism, but I understand if y'all want nothing to do with me 