Here’s another point - if that doppelganger ability works as shown, as Luke Skywalker showed it in TLJ - then what’s to say Palpatine can’t use the same ability to fake a ghost of Luke on Ach-to, which talks Rey into going to Exegol and doing exactly what Palpatine wants her to do?
If Palpatine has been every voice inside Ben Solo’s head, then what’s to say those voices inside Rey’s head as she lay there weren’t the Jedi after all, but Palpatine convincing her to get up and do exactly what he’d asked her to do, in killing him?
How does anyone know that the ritual he was trying to enact requires him to be killed with a lightsaber? How do we know it doesn’t work just as well if she deflects his Force lightning onto him and he dies that way instead?