I often see things about Lavellans that become disillusioned, but my Lavellan actually had a lot of her beliefs affirmed. It probably helps that she wasn't a fundamentalist sort of Dalish, so her beliefs weren't so rigid that learning that they were inaccurate wasn't enough to destroy them altogether. Learning that the elven gods were real, whatever they were, and that immortal elves were real, was literally something of a religious experience for her. (All that time humans were telling her she was the herald of a divinity she didn't even believe in, and then suddenly one of her own deities appears...her whole world was spinning.) Even before that, Solas broadened her perspectives, causing her to both question what she believed and to wonder that even the most fantastic Dalish tales even she had doubts about might be true. Being an Inquisitor was always appropriate for her, since searching for truth was something that had always been important to her as a First and aspiring Keeper. She's still devoted to learning about the past, while trying to make a better future through understanding their true history. And if the Dalish, and her clan, her family aren't accurate to what the ancient elves were, she's still perfectly fine continuing down that path. Culture is built on what came before, and the meaning of things change over time. It was important to her that Solas accept her for who she is. (Thus keeping her vallaslin.) She's already given him a ridiculous amount of trust on all things, and she needed him to be able to trust her in return. I guess only time will tell...
I'm glad you said it, because my Lavellan felt almost the exact same way.
My Lavellan knows the Dalish are flawed and mistaken (especially since I headcanoned that she was city-born and was often called "flat-ear" and "seth-lin" growing up), but she doesn't see that as a reason to condemn, disown, or give up on them. She doesn't believe in throwing the baby out with the bathwater. That is, just because the Dalish are flawed doesn't mean we should discard them completely, throwing out the good with the bad.
My Lavellan sees the Dalish as people who are doing the best they can with the limited information they have. They as a culture try to learn about their past, and that has value even if it is imperfectly implemented. Sure, the Dalish don't know everything about their past (not even most of it, now that people like Solas and Abelas have come forward) and often get the details wrong when they do. But that's part of learning and growing. You learn that things you thought you knew before were incorrect, or learn new details to things you already did, or learn completely new things that you never dreamed of. Just because the Dalish are still in the process of rediscovering and rebuilding the past doesn't mean they'll never get there, or that everything they've gained so far is automatically worthless.
That's why my Lavellan defends the Dalish to Sera and Solas every time they disparage them; not in a belligerent "How dare you insult our heritage!" way, but gently asking, "So the Dalish made some mistakes, does that mean they were wrong about everything?" or "There's so much history and tradition there, should we throw it all away?"
And that ties back into vallaslin. EDIT: Whatever they were used for before, they have come to mean something different for the Dalish today. My character has considered vallalsin such a positive aspect of her heritage, culture, people, and personal identity for so long, she can't see herself discarding everything good about it just because there used to be something bad about it. She sees discarding her vallaslin as symbolically discarding everything good about her heritage just because there's something bad about it as well.
And yes, Agreed with your Lavellan so much on her stance on the vallaslin. (My Lavellan even told Solas, "If you could just look past...")