A couple of other thoughts:

"Because his spirit chose another vessel, out of the hundreds his followers had created. Rey…you’re his clone.”

“You were created to sit upon the throne of the Sith, a vessel for the Emperor’s lingering essence. But what he couldn’t foresee was our connection in the Force. We can kill him, Rey, destroy the Sith…and bring a new order to the galaxy."

To solve the problem of why you can kill his spirit this time if not last time, perhaps instead of ‘lingering’ there you could use a synonym for ‘fading’, perhaps ‘dwindling’.

I’d still like to see it made explicit that the rejected hosts were considered threats (and thus killed) though.