Why did my Lavellan tell him she would help him? Because she was trying to get him to let her go with him, so she could undermine his plans from the inside. And they both knew that was why she said it, because neither of them are stupid, but she had to try. There is a part of her, of course, that desperately wants the elves restored to their former selves, but that's secondary to maintaining the peace (though in a way, how can there ever be real peace for any elf in current Thedas? All of them live under constant violent oppression.).

Which is why he quickly turned it around on her, saying he would not have her see what he becomes - he does not like what he's doing, he knows she will test his resolve if she is there, that he will end up hurting her directly if she is there, and he finds that idea unbearable. She told him she would find another way, and he said he wished there were - not saying it was impossible, but unlikely, if desirable.

I like the idea that my character gets the opportunity to try to save the ostensible antagonist for the next game (even if by proxy with a new protagonist). He's a fascinating character, who I think wants to do the right thing, even if it is not what appears to be the best thing for modern Thedas and its inhabitants. I still think the Veil will be destroyed, regardless - whether by Solas, by Morrigan/Mythal, by Shaper Valta - or by another character who has yet to be introduced.

Honestly, there are so many contradictions between what Solas is doing and what he's done and said in the past, that I'm not entirely certain he's the one calling all of the shots (who might be, if you support this theory, is debatable, though I lean towards Mythal). The current lead writer, Patrick Weekes, said in a tweet that it was a spoiler when confronted with Solas' apparent inconsistent writing, so I'm thinking we don't have even close to all of the information in terms of the reasoning for Solas' actions, and his willingness to potentially rock Thedas to its core, possibly killing even the Elvhen people he wants to restore (as he had so little foresight on the consequences of the raising of the Veil, how can he predict accurately the effects of taking it back down again?).

I refuse to believe that "well, he's just a power-drunk selfish asshole who only told us lies" is an adequate explanation, however, when there is so much else that hasn't been revealed, and there are plenty of instances where he has been forthcoming about his beliefs and knowledge - many of which oppose the actions he may potentially take.

Anyway, I have an Inquisitor who plans to save him, and another Inquisitor who wants him dead.

Both are elves, both have their own reasons for these decisions (mostly propelled by love vs. hate), and both believe these decisions to be well thought out. If Solas had not been written to give the player some reason to be conflicted over the final choice, I think that the game would have lost much of its impact.