Morrigan was raised by an actual elven goddess, and had access to grimore of said goddess.
And she got her existing Eluvian working by stealing a Dalish book.
And the knowledge of ancient elven lore she spouts at the Temple of Mythal is rudimentary at best.
1) Most of what she says are trivia any Dalish could tell her, and which Solas (an actual elven god himself) tells her is not as extensive as she thinks it is; adding that what she mistakes for elven history is actually elven legends and lore.
2) The stories she tells of Creators at the murals are, again, basic legends most Dalish already know, and it's Solas (an actual elven god) who constantly adds more details for her.
3) All she knows that the Dalish Inquisitor wouldn't reasonably know is how to read ancient elven text, but that doesn't stop her from making so many history, lore, and translation mistakes that she herself admits she made after getting back.
Also, she had not the faintest idea the one who raised her was an actual elven goddess, and there's no hint the grimoire of said goddess had anything elven in it, so that's moot.
Merrill was raised by the Dalish, who have a long history of being wrong about everything.
Which part? The part about Arlathan and Elvhenan being just as real, beautiful, and glorious as the legends describe? Ancient elves being immortal? Elves being tied to magic? The divide between spirits and demons not being as cut-and-dried as the Chantry claims? The Creators being real? The war between the Creators and Forgotten Ones? Fen'Harel being the one who sealed them away?
It seems to me they got a lot of stuff right.
I don't think it's really fair to compare Merrill to Morrigan.
You're right. Morrigan found and got a pre-existing Eluvian working by stealing Dalish books.
Merrill rebuilt an entire Eluvian from a single shard and her own research from Dalish sources, and likely could have gotten it working had she gotten the magical password before Marethari interfered.
Clearly, we know who's the superior intellect.