Here there be SPOILERS for anyone who wants to avoid potential Inquisition details.
In Dragon Age 2, Marethari claims that the pride demon Audacity was guiding Merrill into fixing the Eluvian in order to directly possess Merrill and escape its Sundermount prison. We also know that Merrill, despite completing the Eluvian, was unable to make it function.
Based on details near Inquisition's conclusion, we now know the following:
- The Eluvians, much as Merrill and Morrigan have previously stated, represent a travel & communication network. The ancient elves did not have roads between nations or lands because they had the Eluvians. Some Eluvians, apparently, also go to other worlds. The details of that little tidbit are not spelled out very clearly - it's possible that Morrigan knows even less about this than she professes to know - but in the end, going the through one Eluvian takes you to a hub area where one can then exit through a different Eluvian. This hub exists between worlds - not the Fade but similar to it.
Most likely, lacks both the key and the knowledge on how to activate the Eluvian.
It's also highly unlikely that Audacity itself had a key or that it was trapped within the hub realm. This calls into question, as many of have already speculated, everything that Marethari said during the events of A New Path.
I agree with point 2&3. I also think Audacity has a key. I read one of the novels (the last one out, with the Queen of Orlais, not a very good book), and in there, a demon has the key and the knowledge to the Eluvian. He doesn't reveal that it really only works only with Elves, non Elves have trouble naviguating in there.
Merryl has no idea how the Eluvian works and what it requires, wich is why she consorts with demons, thinking she can use them for her advantage.
What I believe you are missing about the doors to the other worlds is this: In the temple mission, where you meet Cory, the Elves' leader states that the Elves destroyed themselves (long?) before the Tevinters came.
So, if they had mastery of the eluvians, if they opened door to other worlds, it's possible they let demons (or something else entirely) into their worlds, wich precipitated their downfall and left them vulnerable to a human invasion. Or it could mean that Elves got greedy with power, thought they were invicible, decided to conquer all the surrounding worlds wich then banded together to defeat them. It's not like it's an uncommon concept in sci-fi/fantasy.