It was beyond unlikely that Palpatine would've survived being thrown down a reactor shaft. Improbable is not impossible. On top of that Lucasfilm reinvents the rules to suit their needs and if Palpatine is ever needed again for the current story they will not hesitate to retcon him into existence. I'm definitely not saying they should bring him back, but they could.He only survives because he performs a transference and sends his spirit into a clone. As far we know a spirit can't exist in Canon, in a any form of limbo. It's either with the Cosmic Force, or bound to an object(like Darth Momin and his mask) or a body. And dark side user can't return from the Cosmic Force either. So unless his spirit went to an object, he's gone, most likely for good. However there are ways of opening portals to the Netherworld. Vader has tried twice to bring Padmé back this way.
So hypothetically, if Palpatine had sent his spirit into Rey. And then she dies, he would have needed an object, probably imbued with the dark side of the Force, or another body standing by, somewhere, on another hidden Sith World.
Looks like she's just straining against the dark energies that end up killing her.
You never let me downLooks like she's just straining against the dark energies that end up killing her.
There's actually a really cool little detail in this scene. As she begins to reflect his dark energies back at him. You can actually see the dark side energies passing through her. Which as I recall is detail mentioned in either the Revenge of the Sith novel or one of the reference books. When Mace does it to Palpatine, it's not just simply reflecting the energy with the lightsaber. But the energy actually has to pass through the user holding the saber. It's incredibly difficult.