I run forgotten realms, and in my own little opinion, any setting, world or whatever that has multiple means of getting from one plane of existance to another; say via a planeshift spell, is and will always have access to any and all gods ever.
I don't know about all of you, but I remember a time when Both Waterdeep and Undermountain had Spelljammer ports in them. I recall Adventures that started in FR, went to Ravenloft, and ended in Dragonlance.
This is a game of high fantasy, where ANYTHING is posible.
So what the cosmology is different, maybe it was a badly worded wish that rearranged all of the planes and forced Mystra to Throw wish outside of wizardly power while all gods everywhere try to fix the mistake?
I mean I'm adapting, why can't you?