Demons don't make deals with mortals unless the demon has something to gain from it. The promise of getting the Eluvian to function was always the hook to eventually possessing Merrill. The demon gave her enough to give her hope and sate her curiosity, and then sat back to wait while its plan came to fruition. Audacity was playing the long game and knew Merrill would eventually come crawling back for 'more' with the Eluvian.
You're assuming that Merril was always the demon's target, which, the more times that I replay DA2, the less sure I believe that's the actual case.
Remember, we have absolutely no idea when Audacity possessed Marethari, other than that it occurs between the end of Merril's Act 2 quest and the beginning of Act 3 (which puts her possession as occurring sometime in the course of 3 years). Was the Keeper completely "herself" and telling the truth when explaining what the demon's plan is to Hawke/Merril? Somehow, I doubt it, I just can't believe that the demon's influence over her only occurs after she is beaten "halfway" and she tries to trick Merril so Audacity can try to kill her. Maybe the demon's intention was to destroy the Keeper? Maybe even destroy the Dalish clan itself or even prevent the Eluvian from being completed/activated? I mean, even before she became possessed Marethari was spreading lies and falsehoods to the clan about Merril after she left in order to 'protect' them from what she saw as evil (the Eluvian shards), and I can't see how her negative words would lessened under a demon's influence. As shown, the taint from the shards were purified (by blood magic, yes, but it was Merril's own blood used, it's not like she was sacrificing people to purify it) and Merril was not corrupted be using them (or by using blood magic, despite what Ander's would claim), but for the Keeper ignores these things and still claims to the clan that she will bring the corruption back.
Marethari was indeed a wise Keeper and can be kind and compassionate.... but she is flawed like any other person - she can allow herself to be spiteful, arrogant, and dismissive of another's (ie, Merril's) views if they're different than her own, as seen in how she handled the Arulin'holm blade situation (rather than keep her word to give it to Merril, as she should have done based on Dalish tradition and values, she instead acts like a child and gives the tool to Hawke, a non-Dalish, instead and explicitly tells him/her not to give it to Merril).
Something else to remember is Merril's words when they reach the demon's bound-shrine - Merril states that that the demon couldn't just escape or be unbound, that even with powerful magic that something terrible and horrible would have to be done in order to it leave (Merril even states to Hawke that she has absolutely no intention of ever freeing the demon in order to learn more of how to activate the Eluvian). So what then is that "terrible" thing that Marethari did in order to release the demon and bind it to herself? I don't know, but given Merril's knowledge on the matter, I doubt it was a small thing...
Also keep in mind that what Marethari reveals about the Eluvians - that they are a gateway to the Fade and Audacity always wanted Merril to complete the mirror so it could use it to escape from the Fade - is utterly false based on the information revealed by Morrigan in the Witch Hunt DLC and in the recent Masked Empire novel: according to Morrigan the Eluvians are in fact portals that connect to realms "beyond the Fade" and in the latest novel, the path traveled between Eluvians is a realm completely separate from the Fade entirely. How then does Marethari come to the conclusion that the Eluvians are connected to the Fade and the demon's plan was to use it to escape? My own opinion? Because that's what Audacity told her, and (from my perspective, at any rate) Marethari was so convinced that what she had been saying was right to Merril (that the Eluvian was always dangerous, etc) that she allowed the demon to sway her thoughts. Just because Marethari is the Keeper and has many years of knowledge and experience doesn't not make her immune to being swayed by a demon's words - hell, she even says that during Feynriel's Act 2 quest ("No one is immune to a demon's offer"). If what Marethari is claiming about the Eluvians is false, why then would she (or really, the demon) say false information of what the demon intended to do with it to Merril? (either the demon is deliberately giving Merril lies about their true nature or Audacity is warping Marethari's existing viewpoints, that the Eluvians are evil, for it's own purposes)