In different circumstances, where she's not surrounded by humans and quietly terrified, she wouldn't have had much to do with him once he shared his opinion about modern elves. My girl is easy going but no matter how interesting the person once they spit out that kind of hate she's done with them. She'll fell for him despite her better judgement and well look at how it ended.
Not really sure what "hate" means though - if it means not necessarily subtle criticism then I hate my fellow countrymen myself, which is simply not the case.
And believe me - I heard my fair share of "you hate us!", "you're tremendously rude and unfair!" even for the mildest criticism, even politely correcting someone on some facts at times.
Therefore I'm just not that quick describe what they say as simple "hate" and I'd like to think neither my Lavellan, especially after she observes and learns how close-minded her own people can be. Like Dorian, she feels a deep need to find the common ground with everyone and she only feels that things like vallaslin serve to divide people.
That's why I didn't romance Sera as a Lavellan. Solas may not like the Dalish but he always respects her faith. Sera does not.
What's more, Solas actually tries to give the Dalish some credit - hence I just don't buy that he only has disdain for them.
As for my Lavellan's rationale for keeping the vallaslin, I imagine her as someone who's very focused on the here and now, elven history being just a source of knowledge that could be used to help all races of today. Her vallaslin is an expression of who she is, so no way he's taking it!
Which God did your Lavellan pick for her vallaslin and why?
... But the same could be told of Lavellan 'expressing herself' by removing the vallaslin.
I'm also not sure why history wouldn't be a source of knowledge that could be used to help all races of today for a Lavellan that removed that tatooo? Past is important for her in that regard, but not enough to define her. In any case, I'd like to think that's one of major reasons why she and Solas compliment one so well as a couple - he's someone who looks backwards, grounding her in knowledge of how things were and she's the one who's looking forward, giving him a fresh perspective (or maybe one he's lost at some point).
And my Lavellan's vallaslin was Mythal's (the smallest one) for very prosaic reasons - it fit the design of her face well (together with the haircut I picked it framed her eyes very well)
Though I do like the fact that her partial, not really that elaborate vallaslin represents her skeptical nature towards gods, worship and tradition.