I think Temeluchus covered evrything nicely. Just two things I want to add.
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.
How Revan delayed the Emperor's plans is explained in the Maelstrom prison flashpoint. After Revan was defeated, the Emperor started using Revan as a means of accessing the LS of the force. As a result of this, Revan was being corrupted by the DS. However at the same time, the connection between the two allowed Revan to slightly impact the Emperor's thought process making the Emperor be far more cuatious than he would have been on his own. This caused the Emperor to delay his plans. Additionaly Revan influenced the Emperor to make the Emperor believe that peace would be in his best interest resulting in the Treaty of Coruscant.
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.
I however am of the opposite school of thought that believes that for story reasons they will never make the Emperor into a operation boss because his corruption ability would make the encounter impossible from a lore standpoint. Also important storyline events aren't supposed to be in Operations, so if he were to be a group encounter boss, a flashpoint would be more likely....although my same reasons for thinking it won't happen still apply. Although I guess either an event in the JK storyline or a seperate flashpoint could result in his corruption ability being sealed. *shrug*