Good discussion. Here are my thoughts.
The Dalish are as they are. No reason to blame BioWare for anything. If that's how they want to portray the Dalish, then so be it. Even if this is something you don't like, it might be possible for the player to change this in a future storyline, where the Dalish are set on the right path by the player.
The Dalish are so far behind in everything they do because of their own choices. They are behind in technology because they stick to the wilderness. They are behind in knowledge because they only pass stories on to those in their own clan. They never pool their resources or share with all the clans. Meaning, humans have libraries and universities where knowledge is secured and shared with all those who seek it. The Dalish only pass along fragmented and often incorrect stories. They are behind in magic because their clans can't have any more than 2 or 3 mages at a time, and they cast the rest out, even if they are children. They are behind in elven lore because they lack the numbers or the means to search old ruins or cross examine information.
In short, elves are so far behind because of how they choose to live. Until that changes, they won't advance. They will only continue to die out.
Why does Mythal keep taking human form? That's simple. Flemeth was the one who reached out to her, and Flemeth is human. Any child created between a human and an elf will always be human. So each new daughter to be birthed for eventual possession will always be human.
Also keep in mind that the same thing that makes the average human more intelligent than the most learned Dalish, is again, how they were brought up. Humans learn about a variety of subjects, and this variety of study is what makes them more capable of learning new things. The Dalish only learn a few limited subjects related to everyday Dalish life. This is why a teenage Dalish boy in DA:I can run off to a shack to try and summon a demon (and get killed), while the human boy is taught how to resist demons and that they should never be summoned in the first place. That Dalish boy was breaking new ground for his clan. No one there knew anything about that, other than the fact they didn't know anything about it and that's why they shouldn't tamper with such things. And so the boy was killed from something he didn't understand or have any hope of defending against.
Again, Ariane and Finn illustrate my above points perfectly. Ariana is trained as a warrior, not a Keeper. So her knowledge of the lore would be limited to begin with, and for a meaningless reason. Finn on the other hand is a beneficiary of hundreds of years of knowledge being compiled by various sources.
In all fairness to Morrigan, she spent like 8 years straight researching the Eluvians. The Dalish don't even know where to look.
Gatlok is a qunari's people's technology. But you wouldn't let a qunari child play with gatlok, would you? The point is that Merril was messing with something she didn't fully understand, whether it was her people's something or not. In human society, such an artifact would be taken to a safe place where many people could work on it, study it, for years perhaps, and test it safely under a controlled environment. Besides, Merril was also messing with blood magic and a powerful demon. It's likely that had it not been for Marithari, Merril would be dead.
I think Dorian's reason was more of what it would say about Tevinter rather than anything he thinks about the elves. Everything he does is to clean up Tevinter's act.