"My speculation > Laidlaw," takes a lot less time to write.
Laidlaw said it was a 'bad call'; that was it. Considering that Marethari ended up setting Audacity loose, became an abomination, and then attempted to kill Merrill, I could see how one could view it as a 'bad call'. However, I'm not willing to condemn Merrill for Marethari's actions.
Also, from what I've heard of DA:I, the mirror does exactly what it was suspsected of doing. Killing a bunch of people.
Marethari claimed the Eluvian would bring back the taint in Act II. Then she claimed it would be used to set Audacity free in Act III, and set the spirit free from it's prison without warning her clan or Merrill about the ramifications of her actions, potentially endangering many people with her recklessness. The Keeper wasn't exactly consistent in her reasons for opposing Merrill's efforts.
Of course Merrill is immune to such things. No way the demon just found a much easier way out. It was being honest with Merrill, but not Marethari.
Merrill learned blood magic from Audacity because she lacked the necessary lyrium to cleanse the shard with ordinary magic, and she didn't set Audacity free. Merrill never claimed Audacity was altruistic in helping her; she makes it clear the spirit is dangerous, which is why she asks for the Champion to accompany her to Sundermount when she suspects Audacity is no longer in the totem in Act III.
The simple difference between Merrill and Marethari is that the former refused to set the spirit free, while the latter did free it in the most asinine manner imaginable. Unless there's anything new about this topic for us to discuss, I'm afraid that's all I have left to say on the manner.