Right can I just point out something here, about Palpatine possibly?
There was such thing as Sith Ghosts in Star Wars Cannon, Darth Bane appeared in Clone Wars and attempts to trick Yoda to join the Dark Side
[Sith Ghost Darth Bane from Clone Wars - Voiced by Mark Hamill]
So whats the odds Palpatine is just a Similar ghostly apparition who is trapped in the ruins of Death Star 2 that Kylo goes to in an attempt to retrieve something? Most likely belonging to Darth Vader such as Vaders Lightsaber etc and disturbs the Ghost of Palpatine, or Rey heads to it, as she is being called by something or someone and she doesn't know who or what it is and much like Yodas encounter with Bane, so Palpatine attempts to stir up Dark Side feelings within Rey.
I spent some time to think about how this would make more sense than an actual clone or body jump etc.