That Marethari might have been the target all along is an interesting theory, although one I don't agree with. But even if Marethari was the intended target it does not absolve Merrill from blame. Regardless of whether the original target was Merrill or Marethari, Merrill was key to the demon's plan. Audacity depended on Merrill's obsession with the mirror, her naivety in dealing with demons, and the stubborn pride that continually led her to ignore advice to forget the mirror, and criticism of her use of blood magic and dealings with the demon. Marethari allows the demon to possess her in a foolish attempt at saving Merrill, which would not have happened had Merrill abandoned plans to fix the mirror and avoided any future contact with the demon.
If Marethari is the intended target than Merrill was duped into being the bait. In either case, Audacity plays her like a fiddle.
My point was that neither Merril or Marethari are flawless in this situation. Yes, Merril was the one who went to Audacity first to learn how to purify the Eulvian, but based on the information given in the story, it sounds like she only went to the demon once and then left it alone (nor was she seeking out other new demons in the Fade to talk to). Also, I don't think I'd called Merril naive with how she views spirits/demons - if anything, she had one of the wisest attitudes of them, based on what she says to Anders after his Act 2 quest and he angry tells her that she could become an abomination:
"Anders... there's no such thing as a 'good' spirit. There never was. All spirits are dangerous. I understood that, I'm sorry you didn't."
Also if Marethari truly allowed herself to be possessed in some vain notion that doing so would save Merril or if even Audacity offered to leave Merril alone in exchange for entering Marethari instead, then Marethari was the naive one if she believe that she could allow a demon to inhabit her body without any serious repercussions. It's true that Marethari wouldn't have gone to the demon if Merril had never sought it out first, but that does not mitigate Marethari's naivety or selfish actions of trying to protect Merril like a child (and I daresay, to even control/influence Merril's beliefs back to being Marethari's, which she always viewed as being right) when the end result was something that she had always preached against and that she knew would end in tragedy and suffering (if not just for herself, but for also Merril or the clan).
Merril is a couple cans short of a 6-pack. She was too lazy to use lyrium for the mirror, so she turned stupid to ask a demon to help her. Idiot. Had she just left the mirror alone or used lyrium, the keeper would be alive and the clan wouldn't be killed, if you choose to fight them.
"Too lazy"? Where the hell is she going to get all the lyrium need to help purify the mirror? Lyrium doesn't grow on trees (if it did, am sure humans wouldn't have to barter like hell with the dwarves for it then...) and the Chantry/Templars controls the market for it, so how was she lazy in not using something which she had absolutely NO ACCESS TO? 
All she had access to was her own blood to power the ritual needed to purify the Eluvian shard/s of the darkspawn taint (and using her own blood as a powersource, I would argue, is not an evil use of blood magic).