I don't quite understand why fans protest against the oh so horrible portrayal of the Dalish as of late.
I'm going to try and address your questions seriously, then.
Apparently, it is terrible that they are wrong about so many things. And yet, there was a big fat clue right in the beginning of the Dalish origin when the narrator says they "cling to their half-forgotten lore".
I actually find it confusing that the narrative seems to frame it as though they got "everything wrong", yet the Dalish knew Arlathan existed, that the ancient elves were immortal, that there was a war between the Creators and the Forgotten Ones, and that Fen'Harel trapped the Creators. That's hardly everything. In comparison, when Andrastians get it wrong - like with the protagonist being met by a spirit, rather than Andraste - the narrative and the character frames it as though they could be right after a fashion. That's the kind of imbalance that some people criticize the writing for.
Did anyone expect the future reveal about the ancient elves to show us that the Dalish were 90% right and just missed a few details? Of course the holes would be much bigger than that.
Again, it's about the presentation. People also addressed the lack of elven content in the Dales, the complete dismissal of the Dalish receiving the Hinterlands in the post-Dalish Boon worldstate, the "three mage" recton with the Dalish, the incessant bombardment of anti-Dalish comments while your comments laud the Chantry of Andraste and the Andrastian faith, ect.
Then we have this huge fuss about the three mages rule. Tons of people crying retcon. Whatever may be the case, I don't think that paints the Dalish as mage haters, but smart and careful.
It is a recton - it explicitly contradicts that magic is supposed to be "dying out" among the Dalish, and that this is the reason why some mages are moved between clans, as Merrill was. We also know that Zathrian's clan had more than three mages (Keeper, First, Second, the halla herder, and arguably the Healer since he notes that the only reason he follows the clan is because he feels indebted to them - and the clan speaks positively about him when you inquire about him); Merrill also mentions that any child who showed magical ability was apprenticed to the Keeper, and she says that it's alien to her that the Chantry would put mages away in Circles where they can't do any good.
As noted, "As each generation passes, magic becomes more rare among the Dalish. As the gift dies out, talented children are moved between clans so that every Keeper has a successor, and no clan is in danger of being left without guidance."
While it may not seem that way in combat, lorewise abominations are very powerful and could easily overwhelm a handful of elves.
The issue of abominations in a respective clan was brought up in Dragon Age II by Merrill, so I don't see why you think it's incompatible with the lore before the Inquisition recton.