I have to wonder if Solas is actually able to kill Lavellan if push comes to shove.
He could kill Mythal, and he ended the world to avenge Mythal. I am thinking the answer to that is "yes."
On to my own thoughts:
Solas expects to die in the course of achieving his goals. I'd argue that he also expects everyone who is helping him closely to die. I noticed in a video if you pick the "omg let me help you please" option, he tells you that his path only leads to death. "This is not a path of honor," I think he says. You know what that phrase comes from? "This is not a place of honor," it's used to mark nuclear test sites to warn people they aren't graves etc. worth looting for the sake of history, but instead places filled with dangerous levels of radiation. How that translates to whatever Solas is up to I don't know, but it doesn't look good. Weekes would not have chosen that phrasing by accident.
I've said it before, and I will say it again: Even if you stop Solas, the Veil is coming down. Sandal's prophecy is not the only one that predicted this. The stone lady did too. I think someone pointed out the creature in the tiny cave also said so. It's also prophesied in the text-based game in the Keep.
Maybe we stop him too late. Maybe we stop him in time but someone else jumps in to finish. Maybe there are agents of other Evanuris among his agents. Maybe someone else figures out his plan and carries it out without him (even if it's just one of his underlings). We don't know.
Team Optimism, take heart. The world won't end. It will change, but there would not be another game if the world ended. Not that it couldn't end at the end of the final game (it could, and then would either end the franchise or lead into a new franchise), but we already have evidence that the Veil's coming down and "everyone will see."
I do expect death and destruction, but let's face it: that's pretty normal in Thedas.
I'm expecting Mythal to be the one who destroys the Veil in the end, because she has told us for three games that she wants revenge. Now we kind of know who that revenge is against. If her Fade scene is any indication, it looks like she may have a specific grudge against Dirthamen in particular--or else Dirthamen is someone she wants to avenge. If I'm right, this could be the turning point in convincing Solas toward a path of redemption. He doesn't want to release the Evanuris; Mythal most likely does--so she can kill them or do something worse to them than Solas did. If he sees that it's likely she will do this if he drops the Veil, he'll want to stop.
I don't want to see the death and destruction that result, but I have to say that I do look forward to seeing the world after the Veil is removed, and seeing what kind of stories are told afterward.