A bit of off-topic I think the pride demon was manipulating both Merill and Marethari so it could possess Marethari.
I've made that argument before in the past.
I genuinely think Merethari was the true target and Merrill was the bait, but if things worked just so then the demon would possess Merrill if the opportunity arose, it just wanted out.
But Merethari was the easier target due to several reasons.
1. Proximity. We know from the short story that proximity affected how much influence Audacity had over Merethari and Merrill. Not only did its influence get stronger the closer they got, but it affected Merethari even more than Merrill before they went to investigate.
2. Pride. Now don't get me wrong, I think Merrill is a very proud person. There's a reason the pride demon targets her over everyone else in Feynriel's dream, followed by Fenris and finally by Varric. But Merethari was also a very proud person who couldn't accept she was wrong about the eluvians, despite not having done any research on them. It's a trait they both share. Merrill felt that her work on the eluvian would help all the elven people, and I've written pretty lengthy posts on how I personally could see the benefit (before Inquisition came out, many of them turned out accurate and was used as I said I would've used them by Briala in The Masked Empire.) But Merrill felt completing her work would make them see. Merethari was too proud to get over her fear of the darkspawn taint returning after the events of the Dalish Warden's origin. She poisoned the entire clan's opinion of Merrill in an attempt to convince her to come back...not sure how she thought that would work, but her worry about the darkspawn taint returning in Act 2 is the sole reason she told the whole clan that Merrill would bring back the taint and that she was a blood mage, as per her own confession in Merrill's quest.
Again, not sure how poisoning the entire clan against Merrill would help convince Merrill to come back to the clan.
3. Power. We know from the devs when they were answering a question on why a desire demon was attracted to Connor, an untrained mage, that demons are attracted to power of all kinds. It's not just magical, but also political, social, and economical power, so long as its power. Connor was chosen by a Desire demon, one of the stronger ones in the hierarchy, because he was the son of a powerful arl and the heir despite the fact that he had next to no magical training and could barely cast minor spells. Merethari not only was fully trained and had focused power, but she was also the Keeper of a clan and thus had very real power among her clan that had nothing to do with her magical training or experience.
4. Lack of caution. From what we can gather from the games, Merrill took every precaution, even preparing her own death should the worst come to pass, whereas Merethari throws caution to the wind in her desire to keep Merrill from doing anything that she (Merethari) feels is too rash or stupid. This also ties into the first case of proximity I mentioned.
After meeting Hawke, Merrill, prior to meeting Hawke made the decision, leaves the clan and is away from Sundermount. She is away from the demon and its influence. According to the devs, Merrill extrapolated information from the single shard from the Brecilian Forest and built her own eluvian from scratch based on what lore and information from the shard she could gather. And because Merrill is no longer in proximity to the demon, she no longer is under its influence. Merethari, who is affected and its also pointed out that she affected terribly the closer they got to the demon, is also the one who says the demon is harmless while remaining in the statue, and then proceeds to remain under its influence range with her clan for years.
It doesn't make sense to knowingly stay in a place you know the demon can influence you to the point you wake up from your dreams in tears and can hear its whispering, even knowing full well that the demon is in a statue. Sure, it's no immediate danger, but that doesn't mean its harmless if you remain in a place where you hear its thoughts and your very dreams are affected for years on end.