Case in point, Meridia used to be a Magna-Ge, but was thrown out by her daddy, Magnus, into Oblivion and is now a daedra. Also, the Magna-Ge are extreme isolationists; they do not like others coming in, so I do not expect any daedra or magna-ge walking in and out of the Aetherius and Oblivion. Also, if they did participate, they wouldn't be in the Aetherius, as the process of participating in creation leaves you heavily weakened, probably too much to make the holes. As far as I am concerned, Magnus was only an architect, and didn't do any self-mutilation that was required to make Mundus, like what the aedra did.
As for MD, I place it under Magnus's desire to end Mundus made manifest. A part of him that broke off and became a new being or something.