Every one of those gods you mentioned are already back. They came back in the Second Sundering. It was basically WotC fix for all the unpopular changes they made in 4e.
But most of those deaths had nothing to do with 4e? Time of Troubles was around as far back as 2nd edition at the very least. What about Moander? Is he back too?
I do like this idea. I have been thinking for awhile now that the Illithid made some sort of demonic bargain to create the tadpoles but I could not theory craft what there end goal might be. That could explain why they went into Avernus to make a deal so that they could rise back into power. I do have some question of there motives, not every god would want to help them. Maybe they are trying to create an avatar for there god Ilsensine?
Pretty sure that he dipped into Avernus because he knew the devils would attack the githyanki and he hoped to lose them there. Reeks of evasive action rather than anything else.
