3. How did Emperor Palpatine survive and return?
Bringing Emperor Palpatine back as the ultimate villain is not a vice restricted only to Disney and J.J. Abrams. In the Expanded Universe now referred to as “Legends” — that is, the books and comics that were written before Disney acquired the franchise — Palpatine returned as well.
In the comic miniseries Dark Empire, Emperor Palpatine was seeking to take over the galaxy again. He had a near-endless supply of clone bodies, and would transfer his mind and consciousness to each one.
The other option would be that his body was recovered from the Death Star, either before it was destroyed or after. If the former was to be true, someone would have had to rescue him from the depths of the battle station in mere minutes (or perhaps he rescued himself?). In the latter, his broken body would have been somehow restored while his consciousness waited to reinhabit it.
None of these options is perfect, and none can be chosen definitively. The clone theory is the option with the least holes, yet we are given no indication this would be possible. The presentation of Palpatine seems to suggest this was his original body, although if that were the case the through-line from “falling down gigantic shaft in a moon-sized battle station” to “safely off the station” seems impossible to track.
This question is begging for resolution via the Expanded Universe. A comic series could show how Palpatine has been manipulating the galaxy from behind the scenes ever since his “death” at the Battle of Endor. Perhaps a novel somehow tells this story alongside another. Either way the manner in which Palpatine survived has serious ramifications for life and death in the galaxy, and whether they can feel safe now that he died in The Rise of Skywalker (again?).