Just to add a little more to the theory, we do have SW precedence of a powerful sith being able to mask his presence and power from other powerful Force users, Sidious did it. Good ole Palps kept himself undetected until he chose to reveal himself and his power; he even fooled Yoda.
Excellent point Temeluchus, this one's the one I agree the most with. One could argue that Revan and Malak meeting the Emperor did have its expected effect on them - they turned into sith.
As for the fight in the Revan novel, I believe they did fight the true body, although it's irrelevant, really.
Now if my understanding was correct, then that would mean that had Scourge not had his vision then Revan and crew would have defeated whatever they were fighting. If they were fighting the true body then that would mean that the only reason that Scourge's vision was true was because he had the vision and had he not had the vision then the Emperor wouldn't have been alive for the Jedi Knight to ever fight, making the vision a self fulfilling prophecy. However if Revan and crew were fighting the Emperor's Voice then regardless of whether Scourge had his vision or not, the Jedi Knight would have still fought the Emperor 300 years later.
After a lot of thought on this (I find myself being slow sometimes XD), I believe it's inevitable that the prophecy becomes self fulfilling
. I have been thinking of multiple "what ifs?" lately, and have concluded that the reason we fight the Emperor is because Scourge acted as he did because of his vision.
He basically paved the way for our encounters (present and future) with the Emperor. Whether Revan, Meetra and Scourge fought the true body or not, Scourge's actions alone are what bring us to the final battle, and what will likely have us beating down on the Emperor again.