Briala managed to get Keys and she isn't a mage, so presumably not
Not located only in the Fade, but the knowledge could be there.
Also, if demons can't comprehend keys, Audacity never would have been able to escape through the Eluvian--not that the Marethari abomination's dialogue makes that idea make any sense either, because she says something about "banish it to the Fade," which would imply that it's bound in the statue in the physical world and not in the Fade at all, which would mean that the Eluvian couldn't possibly be a gateway because the demon would already be here. And if demons can comprehend keys, we'd be getting demons coming out of the open Eluvians anyway.
Marethari wasn't the one making the portal to Hell
Aren't abominations supposed to be stronger than just plain demons? The only way Marethari's plan could possibly have been genuine is if a pride abomination is weaker than a pride demon--not that it could have been genuine anyway because of all the other inconsistencies, but that's a big one.