I’m still pretty sure Palpatine was just himself throughout all of his life. Essence transfer requires that ritual we saw at the end of TROS. And we all know Palpatine only killed his master in his sleep. That was the whole reason he likely did it - his master wouldn’t have enough time or awareness to even attempt a transfer like that.

Back to the Rey background idea. I’m thinking it might actually be a better idea that Rey is still a Palpatine clone/experiment, except she came from a vat like the rest of them. Rey’s parents got drawn into the Sith cult because they’re desperate nobodies, adopt her, and have a change of heart - going on the run. So this way it isn’t the first time that Rey has been adopted.