I don't see it as ignorance, but rather a cultural difference in the perception of the denizens of the Beyond. It's not as though Merrill is lacking in her proficiency because she's Dalish: Merrill is able to discern the nature of the Profane Abomination in the primeval thaig. When encountering Torpor, Merrill immediately identifies it as a creature of Sloth.
You have to admit that Merrill is not the best example of the common Dalish curriculum. You have pointed out several times how exceptional is her knowledge of the eluvians. Since to work in the eluvian she had to deal (not "make deals", mind you) with a demon, it makes sense she investigated more about the subject. Her knowledge in this regard was still inferior to Anders' and in Night Terrors she could be taken in easily by Wryme. Of course, that coud be because she was still a First, an apprentice.
But this is just wild mass guessing. For all we know, a Dalish clan may have a library worth of demonology books. Also thinking too much to find coherent explanations for stupid and disastrous mistakes.