"Seeking for survival, the two spirits, most powerful masters of the Dark Side, did the unthinkable. In a defying attempt to avoid their imminent spiritual death and an eternal suffering in the pits of the Netherworld, the two's essences met with each other. A powerful wave of Dark Side energy irradiated throughout the whole galaxy. A massive shift has been anointed for all the adepts of harmful and hungry forces. A shift of darkness.
'I AM ALIVE! I AM-
THE EYE OF DARKNESS AND POWER! I AM OCOLTUS"
-Two Sith Ghosts United short-story
Valkorion was actually able to physically affect the world while a spirit, to my memory, Sidious was borderline useless as a spirit.
As for Sidious' spirit being kept in the Netherworld by all the dead Jedi, the quote only states that's what happened, it in no way implies that it REQUIRED all of those Jedi to do so.
Occams Razor would dictate that it required all the Jedi since they went there in the 1st place. If something less was required than that would have been used.
Palpatines spirit was capable of possession at the very least, and fly across the cosmos while Chaos actively tried to drag him there, which would explain why he wasn't capable of doing Vitiate esque effects.
This blog is relevant: https://comicvine.gamespot.com/profile/s_w_legend/blog/cheating-corporeal-death-and-abilities-outside-a-c/126610/
Valkorion stomps.
Sheev is extremely vulnerable in this situation.