So she can't really blame Solas for doing what he's doing. He sees no other choice, so he does what he thinks is necessary to achieve his goal (a noble one I might add). She did kill and would kill to achieve her goals as well. But there are things she wouldn't do. Like killing Solas. Or mass genocide (like, killing just everyone in modern Thedas). If she believed it would help the Elves, she would start a war with Tevinter/ Orlais or any other nation. But killing everyone? No. And she can't let Solas do that, because it would destroy him, and al the good he has still left. After all, he doesn't approve of killing people and being cruel. I think that sometimes my Lavellan was feling guilty about her way of dealing with problems (stabbing everyone and all that stuff) and it was Solas who was making her a better person, ironically. And that's why she needs to convince him to change his plans, he needs to be this good person, she fell in love with.

About the questions:
I have no idea why destroying the Veil is an insta-kill button. I don't understand why this has to work that way. It could cause some disasters, a lot of deaths, people would feel lost and they would have to adapt. But it would also bring some good I think. But I may be just ignorant. Solas should know everything about the Veil, after all he created it, but I want to think that he does not in fact know everything, and is wrong here. Maybe Mythal knows more? Maybe she's testing Solas to see how far he would go to save Elven People? Mythal is known to do things like that, after all. Although 'You know not what you ask, child' line doesn't make me very hopeful.
I wonder what were dwarves like before the Veil, but it brings me a headache. Again, no idea. Humans? Where were they before? This is strange, it's like they never existed before. Hmm... I think they lived on another continent, or maybe they were another experiment of mad scientist Ghilan'ain.
Another thought about the Veil: i'ts brought up multiple times, that there are more mages born than ever. Fo many years, no somniari was born and yet we have Feynriel in Kirkwall. Something is happening to the Veil already. It might be going down gradually, but Solas would know about it for sure. The magic seems to be coming back anyway. The dragons are awakening, there are more of them than before. What's going on with Thedas, really?