Also, Merrill making a deal with Audacity isn't a flight of fancy or fanfiction. Merrill made a deal with Audacity to learn how to use her blood magic. The lore states that blood magic is so rare in Thedas that most mages learn it from making deals with demons. Now, granted, there could have been books around she could've learned from, but she says it told her how to cleanse the mirror with blood magic. It told her how to use blood magic.
Also, Audacity knew that she would ultimately fail with the mirror and knew that she would PROBABLY come seek its counsel when she exhausted all other sources. That is probably when the demon would start in on its, "Oh... well... If I was freeeee I could help you." It was basically playing the part of mystical drug dealer, "Don't worry, Merrill, the first one is free..." Marethari knew this better than Merrill did.
Anyways. Just because it is a point of characterization you don't like, doesn't mean you can ignore that she flat out tells you she dealt with Audacity to learn how to cleanse the mirror.