Then why does Merrill say that the Dalish have always said "we're the only true elves" and that's why she didn't even really pay attention to the elves in the Kirkwall alienage, although she lived there for years? Why does she call Feynriel a "half-breed" and wonders if the Keeper will accept him into the clan? Why do some of the Dalish elves and their clans treat the city elves as lesser?
Merrill never claims that the Kirkwall elves aren't true elves. Merrill refers to them as elves in Act I; she laments that Tevinter elves don't try to escape from slavery to find sanctuary with the Dalish. "But if they ran away, the Dalish would help them." She also thinks Kelder should die to protect the elven children who he has been murdering with near impunity. She argues with Fenris in Act II that he should care about their plight (and even refers to them as "our people" in the conversation with him), because their situation matters.
As for Feynriel, Merrill supports Hawke advising him that he would find sanctuary with the Dalish, and she opposes making him tranquil. She verbally opposes selling his soul to Torpor. There's also the fact that Merrill is willing to enter into a romance with Hawke - a shemlen - despite the fact that some would disapprove of the union, and she knows all their children would be human.
Furthermore, after the Andrastian elves were left homeless in the wake of the war, Merrill decided to look after them and take care of them, even if it meant separating from her romantic partner Hawke.
That aside, I'm not claiming that all Dalish are the same; I'm pointing out that the way some of them act doesn't reflect on the views, personalities, and attitudes of the other men and women of the People. They are a diverse group of people.
... Which is the exact opposite I'm doing, since I've been speaking about "some of the clans".
Again, these opinions exist in the Dalish culture - and from what I've seen, they aren't all that rare, either. The point being that they have defined their culture this way and come up with insults to reinforce this divide (again, the very existence of the slur knife-ear is kinda testament to th). It's true that religion might be one dividing factor, but then again Pol was allowed to join Merrill's clan and he converted - what is to say the Alienage elves wouldn't do the same? And even if they did want to keep their Andrastian religion, so what? Should inclusion to a society come with a forced conversion?
Being Dalish is a life of hardship; it's not an easy one. There's primarily the factor that Andrastians typically threaten them because of their religion. The Dalish also aren't a pan-elven movement; they are aimed at trying to maintain as much of their culture as possible, and they are religious as well (even the ritual to acquire vallaslin is steeped in meditation on the ways of the People and reflection on the gods).
I think a proverbial homeland would be a different matter (hahren Paivel mentions his hope about Dalish and Andrastian elves learning from one another in the Dalish Origin, particularly his notion that Andrastian elves could help the Dalish learn to co-exist with humanity), but the Dalish way of life is all about striving to maintain their culture and religious beliefs. It's a voluntary choice to become Dalish or join the clan, as we see with how Zevran's mother, young Zevran, Arianni, and even former Firsts like Velanna and Merrill voluntarily left.
The difference here is that the city elves have no power inside the society they live in and don't wilfully define it the same way the Dalish do. The humans are the ones who are spreading the rumours about the Dalish as blood-ritual-inducing, baby-stealing and murdering savages, because having elves running away from the Alienages to join the Dalish is the last thing they want or need. So the city elves have a very one-sided view of the Dalish not because they chose to or because they view their existence as some sort of failure of "elven-hood", but because all they hear about the Dalish is the humans' propaganda, and they hear very few dissenting views on the issue.
The Dalish clans that choose not to give aid or accept "flat-ears" and who keep telling their fellow Dalish that they "are the only true elves" actively create and reinforce this view of the lesser nature of the city elves by their own volition. That's a completely different matter.
Again, the Dalish have it rough, and the elves in general are kinda screwed all over Thedas, which sucks. I feel for the Dalish, I really do. But if punching down is a way some of the members of your culture use to uphold you culture, something is very wrong - particularly since these attitudes keep popping up constantly in the games and in the books.
You do realize that Dalish clans are typically driven off land (as mentioned as early as the Dragon Central days, as well as in the Dalish Origin, and even Clan Lavellan roamed the Free Marches because travelling between the territorial boundaries of the city-states meant that the humans would be less likely to try to attack the clans since it might provoke one of their neighbors). Clan Sabrae also faced threats to convert to the Andrastian faith while they were on Sundermount, which would be an issue.
There's also the issue with the templars, who are a threat to the clans. Merrill says templars are part of the reason why the clans are nomadic (and likely the reason why some clans limit their mages to three), and she advises that she's been cautioned to avoid using magic where templars might see. Even Ariane protected her clan from a templar. "The Keeper that leads each clan also further increases tensions with the Chantry. Apprenticed from a young age, Keepers maintain the traditions of elven magic and pass down their knowledge in turn. The Chantry regards all Keepers as apostates, mages who operate outside of the Circle of Magi, but unlike rogue mages from human society who are quickly hunted down by the templar orders, the Keepers are not so easily taken. Pity the templar who takes it into his heart to track the Dalish into the forests, thinking to take the leader of a clan prisoner."
Also, I'm a little confused why you bringing up the views held by some people, and why you persist in acting as though this marks something wrong with the entirety of the People. They're an ethnic group; a varied group of men, women, and children with different ideas, perceptions, and views. Even Solas comments on this. It's why Clan Virnehn refused to help the Halamshiral elves, while Clan Lavellan were willing to give their lives in defense of the Wycome elves who were threatened by the impending Marcher humans from the neighboring city-states.