im honetsly curious why its so bad people want to leave Faerun.

The part about killing and replacing most gods though? that will be a bit hard, what with the parasitic relation they have with the world.

Best methold would be probably beheading the dragon, literally, as the start, just so, there wont be heavy interference.
as for the rest of the gods? i dont even know most of them, so SOME may be rehabilated (maybe the nature gods most likely?) though there will be opposition, maybe even more so as not everyone of them will die (so if you wanna spare Mystra, you gotta do it smart, so the idiots dont kill her, force her to fight or use as a shield).
...again i dont know enough to jug who needs killing and who not, so i leave it to others to judge.

aside from that, im swayed by the "hug Mystra" club, so yeah, heads up and hugs for her.