Will Merrill still be a true believer in the Creators though if she learns the truth about them?
Merrill was never a "true believer." She always showed signs of agnosticism, being open to the existence of various deities but not firmly believing any of them are fully true, but choosing to worship her own people's deities rather than the ones humans try to shove down her people's throats with a toilet plunger.
As she told Anders, "Your 'Maker' is a story you humans use to explain the world. We have our own stories. I don't need to borrow yours."
Also, when Hawke says of his dead mother, "She's with the Maker now," she gently quips, "Or Falon'Din. You never know."
She was never depicted as absolutely convinced her people's religion, legends and stories were 100% right based on indisputable fact (unlike the Chantry). She constantly says "Our stories say X, Y, or Z," "Our Keepers tell us X, Y, Z..." which leaves room for alteration. If she was told the truth behind the stories, I think she'd keep an open mind, and possibly even be relieved to know the real reasons behind many legends.
No one is claiming Merrill isn't a three-dimensional character; some of us are simply pointing out that it wouldn't be in-character for her to trust (and therefore work with) Fen'Harel.
I'm sorry LobselVith8, but I have to disagree with you on this one.
Once again, she tells Anders, "Your 'Maker' is just a story you humans use to explain the world. We have our own stories. I don't need to borrow yours."
So if an ancient elf claiming to be the Dread Wolf told her the whole story of their people's history, I think she'd hear him out. She's not so dogmatic that she'd say, "No! Our stories / Our Keepers said you're a wicked trickster trying to lead us a stray, so I won't hear you out, demon!" She herself is the first to admit many times in DA2 that her people's history, lore, and stories are missing so much information, and that their religion and lore are largely stories to explain the world and their place in it (though she believes the same of every religion and world mythology). If told the missing information of her people's stories, and Fen'Harel proved some of the truth of it (like the Shattered Library) and his sincerety, I think she'd be willing to hear him out.
Granted, I think his plan to destroy this world to rebuild the old one would not work well with her bleeding heart, since she's so compassionate and has made many friends with non-elves (like Hawke, Isabela, and Varric), so I can't really see her siding with him to destroy all non-elves just to restore the elves.