I dont think Revan would have made a mistake with who he fought. I think the fight with Revan is also explained in the book that they delayed the Emperor's plans. Who knows, I do think that the Emperor can be killed, but it could be similar to Sidious who was able to transfer his life presence and abilities to other people or clones.
Revan could very easily been mistaken on who he believed he was fighting. The Emperor's Voice contains some of the power and essence of the Emperor and given how powerful the Emperor is, it's easy to see how a sampling of his power in the Voice could be mistaken for the Emperor's true form.
Fighting with Revan did indeed delay the Emperor's plans, but that doesn't provide direct proof that Revan fought the actual Emperor.
We have evidence that it is possible to stall the Emperor and his plans via the JK story. We know the JK fought the Emperor's Voice in the end and killed that vessel, the shock of the Voice being destroyed while unprepared severely weakened the Emperor's true form and has put his plans on hold until he recovers.
Apply that scenario to Revan,Meetra and Scourge's fight against the Emperor and it seems more plausible than a scenario where they fought the Emperor's true form,survived and Scourge was somehow deceived into thinking the Voice the JK killed was the actual Emperor.
The Emperor can transfer his essence to other bodies and he can be killed. In fact, I and others believe,he will be killed as an ultimate Operation boss in a future expansion. The Emperor is too dangerous to both factions and the galaxy as a whole to be left alive and he is too important of a villain to have been killed off in a single class's storyline.