Each Trilogy happens within its own self-contained universe.
In light of this, the Emperor in the OT has no Force sensitivity but has merely cultivated an illusion of its power by the use of simple tricks and nonsense in order to win and keep Vader under his command.
-All of his ‘predictions’ in ESB and ROTJ are simple extrapolations where they are not him simply claiming that things Vader had just told him had been foreseen.
-His unlocking of Luke’s cuffs could be done by remote.
-His lightning is identical in look and function to a simple ion beam such as the one used by the Jawas on R2, and such a weapon could be easily hidden in his robes. The beam seems to work only weakly on the biological Luke but almost instantly fries the electronics of Vader’s suit, just like a droid-disabling weapon. Nobody else in the OT uses Force Lightning, not even the powerful Vader, which could be further evidence that it is merely a trick.
-When he is thrown down the reactor shaft, the resulting explosion could be from his power source exploding or from striking some sort of reactor on the way down.