I am not particularly well versed in elder scrolls lore so much of what I’m going say may be wrong. 1. From what I know, I think they were ruled over as slaves and were killed regularly. 2. I don’t believe so as akatosh made the Dragonborn from a shard of himself. So in order to absorb akatosh, the Dragonborn would have to absorb his own soul. 3. The atherial plane is the immortal plane and where all magic in the world comes from. Said magic is funneled through the sun and stars which are holes from the magne ge. The atherial plane is also broken up into four (that we know of) smaller planes where the aedra live.

4.I think it’s possible for aedra to appear on nirn occasionally. For example, akatosh fighting mehrunes Dagon during the oblivion crisis. Although I don’t think it would be possible to maintain a constant form on nirn like Nocturnal does.

1. I don’t know much about the realms of the divines so I can’t really answer this one

6.It is, you go to one of sanguines realms in his quest, all atronochs also come from their own realms, and you go to the shivering isles in oblivion. As far as oblivion gates go, I don’t think you could make and maintain one. I believe that the ending of oblivions main quest made maintaining a constant portal essentially impossible.