Plaintiff wrote...
No information is given to suggest that Merrill thinks this is what's going on, however.
In that particular quest? No. In other quests that deal with deals with Demons? Yes. That's her entire premise on dealing with Demons: play them before they play you.
And on another occasion when Hawke tried to smooth talk his way past some ordinary human guards with a very obvious trick, Merrill blew it. If she thinks this is a trick too, why does she suddenly have the good sense to keep her mouth shut?
To be fair, that instance you're talking about is in Act 1 long before both the Deep Roads Expedition and Night Terrors.
During the Deep Roads, before Hawke says anything to the Hunger Demon regarding the deal it proposes, she'll offer her advice on the matter:
"There's nothing to be afraid of. You can use it, if you're careful."
That says that one can indeed trick Demons before they trick you.
During the Act 2 quest, Torpor suggests the possession of Feynriel she will voice her doubts on actually letting him do such a thing.
"I'm not sure we should be doing this..."
If Hawke agrees, she also reiterates her warning of caution from Act 1 more or less and says "Be cautions. Demon's don't always keep their deals".
Much like how Merrill overrides Fenris and Varric, I believe Anders will override Merrill's comments
If Merrill is present when the quest is given by Arianni, Merrill will say, quite bitterly "The Keeper is delving into the ancient magics for this half-breed? She wouldn't do it for me."
Actually, it's not bitterly said. It's more depressingly said, filled with sorrow at the thought of how Marethari's being hypocritical for Feynriel and will delve into ancient lore for him but won't for her. She's more sad then angry, but I'm certain there's a bit of anger in there.
See, think of it like this: Marethari's going on about the "dangers of how Merrill's going to be possessed" and how "The Eluvian is evil". The Eluvian is a part of the ancient magics of Arlathan. Merrill at first approached Marethari for combined aid in restoring the Eluvian and cleansing it of the Taint, to which Marethari said "No!", prompting Merrill to turn to blood magic to have any hope of accomplishing said goal.
But then Marethari will go out and research other ancient magic about other things that carry far more danger then a mirror fragment. Worse still, she's apparently willing to travel miles and miles to help strangers to the clan, but can't even take the time of day to assist Merrill?
It's not really that she hates Feynriel, it's that she's mad at the hypocrisy of Marethari, which prompts her to use an insulting term in reference to Feynriel. She's emotiona. People say things when they're emotional, sometimes things that aren't in their character. Somniaris are noted by Marethari herself to be incredibly dangerous and incredibly powerful, but she's still researching ancient magic to help him. But Merrill can't even get Marethari's assistance on the Eluvian even after it's safe.
Furthermore, Merrill approves of Feynriel going to the Dalish for aid in Act 1, so she doesn't actually want him to be harmed.
I think Merrill supports making the deal with Torpor because she resents the attention Marethari is giving to Feynriel.
She does resent the hypocrisy Marethari is engaging in, yes. But that is not why she approves of the deal.