Darth Godzilla
Your Lord and Master
I don't mean to assume too much, but Sidious might not have wanted to kill them outright so that Anakin would see the battle and be converted to the dark side. I don't want to start a fight on that, though.
Anyway, Vader and Sidious take this. The instakill ability is not definite. Vader is a MUCH more powerful Force-user than whom the instakill was used on. If people can argue in the Vader vs. Dooku thread that Dooku can block Force Choke even though we've never seen it blocked, I argue here that Vader can block the instakill.
Hey, why doesn't Vader use Force Crush? That'd immobilize her. It's practically as definite as an instakill.
Anyway, Vader probably beats Traya in the Force due to his Dark Side mastery. If it even gets to a duel, Vader rips Traya into tiny pieces of cauterized flesh and bone. Then he goes over to help his master; Revan/Sidious is going to be a longer fight either way. Then Vader kills the already weakened Palpatine and claims the title of Emperor for himself. He converts Luke to the Dark Side in the end, gaining a new apprentice, and with the help of his son, overcomes his limitations to reach his full potential. The Empire reigns supreme. When the Yuuzhan Vong invade, the mighty Empire crushes them with a series of victories. The galaxy belongs to the Chosen One and his son.