Merrill seems perfectly capable of understanding how dangerous spirits are, especially given her ability to identify the type of spirits Hawke encounters in the Deep Roads and in the Beyond, so I can't agree with your opinion on the character.
Saying "spirits are dangerous" doesn't mean that she actually understands how spirits can be dangerous. She is blatantly incapable of actually grasping that a demon's plan can involve anything other than the thing directly in front of itself as a target - she treats them like dangerous animals, not dangerous people.
She's the prime example of someone who uses the right words without actually "getting" it.
The fact that Merrill addresses the Profane Abomination as a hunger spirit, and Torpor was a spirit of sloth, would suggest that while they view all spirits as dangerous, they aren't ignorant about the characteristics of spirits. They simply don't divide Spirits and Demons into categorizations of Spirits being the First Children of the Maker, and Demons as Spirits who turned their backs on the Maker and embody different "sins".
That has nothing to do with what I've said. Merill can't understand that demons are cunning. She can't even wrap her head around how they are cunning. That's why Audacity played her like a fiddle: because at no point did Merrill ever consider that she was anything but the creature's prime target and, in fact, it's only target for possession. The idea that Audacity was just using her to manipulate Marethari didn't even cross her mind.
If Marethari releases Audacity and falls prey to it's machinations, then I'm going to place the blame on Marethari, not Merrill. I also don't share your opinion of viewing her as "incapable" simply because the Keeper fell prey to Audacity's deceptions. Merrill was living in Kirkwall, and as she points out, the clan should have left years prior to Act III. I don't expect Merrill to be psychic, or to expect Marethari to behave so recklessly with her life, and the lives of the men, women, and children who are under her charge.
Merrill had a naive, childish understanding of the situation: Audacity is only targeting her, the only thing it wants is to possess Merrill, and all that Merrill has to do to keep her clan safe is to keep herself away from Audacity. The idea that Audacity is actually targeting both her and Marethari, that all it wants to do is to keep the two separate and to try to manipulate their ill feelings toward one another to score one of them as a meatsuit is completely beyond them.
Merrill isn't at fault because Marethari let Audacity ride her, and I've never said that. Merrill is at fault for, as I've said repeatedly in this post, for not being able to understand or even consider the very sophisticated game that Audacity was playing.
There is nothing that requires Merrill to be psychic about Audacity's intent. It's literally using the most basic and elementary form of divide and conquer possible. But since Merril is incapable of wrapping her head around social politics, she's entirely incapable of dealing with any demon more complex than a rage demon, whose basic mode of thinking is "Hulk smash!".
Merrill failed, comically and miserably, in understanding the plot that Audacity was using. She failed to take any steps to counter that plot, and instead played right into its hands and enabled its schemes. She was such an incredible patsy that she enabled a demon trapped for millennia to escape. This is the highest possible order of failure when it comes to bargaining with demons.
I've continually pointed out that the clans are varied and different, as the developers themselves have pointed out about the Dalish as a people. I've also never said that the Dalish were perfect, since I've pointed out elven characters who have made mistakes, from Zathrian to Marethari. I'm not willing to vilify all of the Dalish clans all across the continent simply because of the behave of a few people.
The lore, on the other hand, has continually pointed out that the Dalish are quite similar in several important ways, particularly in their hypocrisy and xenophobia. Seeing yet another hypocritical and xenophobic Dalish tribe is not a lore breaking moment.
This isn't about vilifying the Dalish - this is about calling a spade a spade.
We have Zathrian, a single person who doesn't condemn his clan simply because of his actions; we have Velanna, who was exiled from her clan for her behavior and actions; we have Marethari, who acted without the knowledge and consent of the others among her clan. I'm not seeing how these individuals should condemn the Dalish as a whole.
Only in your head have I condemned anyone. I said that we've seen 4 out of 5 mages deal directly with spirits, contrary to the ostensibly "no spirit" stance of the Dalish. This isn't about blame. This is about literally stating facts that we've seen directly in-game or in the books.