Your theory hinges on “The Emperor is sooo powerful/evil that he…” which is lamer than shit.
Or maybe, just maybe, the prequels are shit. Have you ever looked at it from that perspective?
It doesn’t help in this discussion. It’s a “Because not OT”, “Because not George Lucas” and “Because it would diminish the OT” type of response. It definitely doesn’t work when there’s a simple as shit reason for why Vader can’t use Force Lightning.