Harriman was not an experienced Keeper whose culture specifically prohibits magic involving spirits. The Dalish do not have the luxury of Templars and Harrowings, and thus the Keepers must be carefully trained to resist demonic influences from the moment their magic is discovered. I don't believe that she would have been easily influenced by a demon, no matter how long she remained in proximity to it. Her behavior is also consistent for the first two Acts, but I will admit that we have no way of knowing what happened between the second and third Act due to those asinine time skips. Also, the varterral wasn't even that powerful... I mean, sure it was probably too much for the Dalish to handle, but Hawke and company were seasoned warriors at this point... it's not like it was that risky... do you think it would have been better if Marethari had simply given up and handed over the tool rather than make one last attempt to change Merril's mind? It seems to me that regardless of whether Marethari was right or wrong, all of this interference in the life of her former apprentice shows that she genuinely cared for her... unless you have a different motive for Marethari's actions?
I haven't read past this post, a few pages back, yet, but here's my take.
Merethari is the Keeper of the Clan. I get that she acted with the motive to help Merrill, and accept that.
But it is my opinion she went about it all wrong. She fully admits that she told the clan that Merrill became a blood mage, and outright says that Merrill "will bring back the taint" through the eluvian that she is building from scratch, thus turning the clan against her.
And then asks her to return after admitting to slandering Merrill's name.
She doesn't give a reason in Act 1, she barely knows Hawke and only asks us to take Merrill with us. Merrill says that she can be a face in the crowd in the alienage so she won't be easy prey there. In Act 2, Merethari says that Merrill will bring back the taint, and that the eluvian in the Brecilian forest cost them two hunters. Merrill also brings it up. If the Warden is Dalish, Merrill will admit that she originally wanted to find Tamlen through it.
But Merethari also refuses to look into any lore, which another clan had as they had a book all about eluvians, which Morrigan stole in Witch Hunt, saying that the ancients wanted the technology lost, without a lick of proof to back up such a claim.
Then in Act 3, as an abomination, says that Merrill would summon a demon, which was already sundered from the Fade by the way, and said demon possessing Merethari then tries to trick Merrill into thinking Merethari was cured, and if Hawke falls for it, stabs Merrill.
Merethari's reasons for opposing the eluvians change over the course of the game, and that makes her lose credibility. Her actions also put the clan at risk.
Yes, she cared for Merrill. But as the Keeper, her duty was to her clan first and to Merrill second.
It is my belief that Merethari had skewed priorities and her experience with the eluvian in the Brecilian Forest made her not trust them at all to the point she didn't want to do any research on them. She put Merrill over her duty to the clan, did not do the research, and as such, does not have the credibility to tell Hawke or Merrill what will or will not happen.