G'raha is looking at the big picture, not his personal desires, and submitting to the will of ancient Allag was his way of preserving hope for the world.
"What if the world's hope is different from your hope? Even if the world is [safe], it's meaningless if you aren't happy."
We endure the despair and keep fighting. That's why we're the hero of the story.
1. Maybe, but I felt the Crystal Tower was intended to be a standalone side-story. The Garleans were able to get the Ultima Weapon up and running again, though, so it's not outside the realm of possibility; it's just highly improbable, especially with the current technology available to us.
2. Same as 1, pretty much. It's also possible nobody will ever open Syrcus Tower again, ever, but that's why it's "hope" - we have to believe, not know.
3. Nobody, really, but again, "hope."
4. Need to rewatch the cutscene, but I don't recall Doga and Unei telling him it was only temporary. Either way, once / if people are able to reopen Syrcus Tower without the "key" it stands to believe they could operate it without the "key" as well, and even if not G'raha could probably have children able to control the Tower now that his Allagan blood has been enhanced.
All just possibilities, though.