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.
The problem I've continually addressed, is that Audacity isn't in the Fade; it's not like the situation with Connor and the Desire Demon. Audacity is trapped, and completely sundered from the Fade.
Merrill learned blood magic from a trapped and impotent spirit, and several years later, it's still bound to a statute in Sundermount. Even Marethari never suggests that Merrill had to make a deal with Audacity to learn blood magic; the Keeper argued that Merrill restoring the Eluvian was Audacity's way to escape, which is why Marethari freed the trapped spirit.
There's also the simple observation that literally no one in the narrative claims that Merrill had to make a deal with Audacity in order to learn blood magic. Absolutely no one.
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.
What's funny is that Marethari doesn't claim this at all. Not in Act I, II, or III. The Keeper claims Audacity will escape through the Eluvian, which is why she released and sealed Audacity inside her.
It's also funny that this is completely different than her initial reasons in Acts I and II for opposing Merrill's construction of the Eluvian, which was about bringing back the taint that had already been cleansed from the shard (hence why she's not a ghoul like the corrupted elves in Witch Hunt who were exposed to the remaining shards of the Eluvian).
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.
Merrill said she spoke with Audacity to learn blood magic, and WoG addresses that she constructed the Eluvian from the lore she gathered and the information she extrapolated from the shard.
No deal is ever mentioned by anyone. The fact that this is never stated or implied by the narrative, and what the characters explicitly say, is the primary issue here.