I think maybe you're underestimating how big of a challenge changing cultural ideas are. Yes, you're right, on an individual level there will be some Dalish that are more open to change than others but as they are now the Dalish are an incredibly insular society. Most of the clans interact very little with other people - even other Dalish - and likely are not accustomed to integrating new ideas into their worldview. The entire reason Merrill becomes a pariah of her clan is because she is challenging the traditional beliefs of the Dalish by fooling around with blood magic. The elves in DA:O never bother to question why their Keeper remains immortal while everyone else dies, despite the fact that he's never shared that very important information with anyone. You have exceptions - like Lavellan's clan - but on the whole we're not looking at a flexible culture here.
And in the real world, it takes a very long time for cultures to change, and the changes are often not undertaken smoothly. Especially when religion is involved. Heck, you can't get most governments to agree that they committed atrocities that there is actual physical evidence for. Even in free democracies. People do not want to believe that their ancestors were wrong, because it usually means that their own worldview is false in some way (i.e. that they've been a dick their whole lives for no actual reason). I don't think that the Dalish as a whole would accept the say-so of anyone if it were to reflect negatively on their culture. Especially when their whole identity is built upon the premise of being a free people driven out of their idyllic homeland by evil shems. Right now they think their history was taken from them, and it'd be a completely different game if they figured out that the elves actually messed it up for themselves and it was only later that the humans came and did all sorts of horrible stuff to their culture. I think they absolutely romanticize their culture - you can hear it in the way they talk about the past as if it were "once upon a time" where everyone was immortal and lovely and sparkling.
And I think maybe you're underestimating just how fluid cultures really are. By their very natures, cultures constantly change, compromise, and adapt to new situations, surroundings, and/or generations. Trust me, I'm studying Jewish and Native American history and cultures in-depth this semester. Holy Crud, you would be surprised just how fluid and flexible they are just by their very existence.
You would be surprised by how many many "timeless traditions" many cultures have that were often created out of necessity or in response to changing circumstances. Just look at the Dalish. When they got their new homeland from helping Andraste, they dedicated their new society to recreating Arlathan. When they lost their homeland, they became nomadic out of necessity to stay out of arm's reach from humans. Solas himself admits that each Dalish clan lives also varies from tribe to tribe. Solas himself admits that no two tribes are exactly the same. Some trade with humans, some are little more than banits, others have disappeared into the forest entirely.
Anyway, I think you're missing my point. I personally don't think the Dalish want to learn about or recreate Arlathan culture because they think it's Perfect, Flawless, and Utopian. I think they want it because it's theirs. It's elven history, heritage, traditions, religion, and culture. They've lived under the boots of humans so long (first under Tevinter and then Andrastians) and have so little knowledge of their own history and culture and way of life, and so would rather learn about and recreate their own society rather than continue to adopt the human's. As a culture, they want to learn about their past because they know so little about it. And if what they learn isn't what they wanted to hear? Oh well, that's part of the learning process.
Sure, they romanticize it a bit, but I don't think they're all so steadfast in their romanticized view that all of them would go bananas if they learned something a little less than ideal about it. I keep seeing people acting like the Dalish are trying to learn about the past because they think it's Perfect, Flawless, and Utopian, and that learning something inconvenient or upsetting about their past would... what? Shatter their delicate worldview? Cause them to react with violence, mass riots, mass suicide; what? I personally don't think so. Yeah, it would be hard to swallow for some Dalish at first. Some would be in denial, some would reject it, some would need time to think about it, some would take it into consideration as they kept recreating their past, some might take it in stride. Some Dalish Clans might adopt this new information while others might branch off into more conservative groups.
That's my view though. I'll say it again though: It annoys me that Solas, Mythal, and Abelas know what ancient Arlathan was like, yet look down on the Dalish for not knowing, yet they don't share that information. Solas seems to have decided their reactions for them, Mythal could care less (she cares about her revenge more than her own people), and Abelas is... Abelas. As far as I'm concerned, if you have a match but choose not to strike it, you have no right to judge people who grope around blindly in the dark. Rationalize all you want "They would react badly to seeing fire," but as long as you don't show them the flame, you don't know how they would react. And if you're not going to help, you don't get to sneer at them for not being smart enough to think to strike a match when there's one in your pocket.