This is how I described essence transfer in chapter 1:
"…willing your spirit on death to transfer into the killer. It only worked if the killer was harnessing the dark side and the victim was defenseless. In such a case, the killer’s spirit would be forcibly removed, and the victim, now the offender, would enter into a new host body. "
Now, the reason I HAD to make it this way is because otherwise Palpatine would have no reason at the beginning of the novel to NOT let Kylo kill him. If essence transfer were described in the way that you have it, then Palpatine could possess Kylo at the beginning without any risk whatsoever. He would go and fight Rey, and even if she killed him, he would simply possess her instead.
In that chapter, I placed particular emphasis on Kylo figuring out why Palpatine didn’t simply possess him at first. Under the description provided above, it makes sense. If Kylo’s body, with Palpatine’s spirit possessing it, engaged in a battle against Rey and died abruptly for some reason, Palpatine would both be unable to possess Rey or return to a clone body. So Kylo figures out that the reason he hasn’t possessed him yet is because Palps wants him to do the dirty work of killing her. He also figures that it’s possible he wants them to fight so he can figure out which one of them is more powerful, and therefore a more worthy vessel for him.
EDIT: Should also mention that I discuss how he is in a clone body (a more advanced form of essence transfer with dormant clones). But that isn’t relevant to this conversation.
The idea I had involved, well…
In this version, Palpatine wanted Kylo Ren to be willing to kill him, but Ren is still conflicted, so Palps hopes that by killing Rey, the one other person Ren is close to, it would cement Ren into the dark side and he would become more willing to kill him under his orders without any hesitation, he would not refuse to kill him since he would no longer be held back by his goody-goody feelings.
EDIT: Oh yeah, I forgot Kylo Ren still does evil acts willingly by TRoS…