You wonder why Dooku can't figure that out for himself. The only way two Sith can ever be relatively safe from the others' machinations is if they have an intensely strong personal bond. Seems like Dooku and Ventress had that bond, which maybe is why she didn't predict his betrayal.
From the book, he thinks he has time because he's so important to the war and he hasn't learned everything he wants to from Palps yet. He also wasn't 100% sure of Ventress due to her annoying tendency to fail, the same seemingly true for this episode. Given that Dooku was able to deal with her and the nightsisters while half-asleep and blind, they would not have stood a chance against Sidious.
Maybe where this story is going is to establish that Sidious is looking for a young, strong replacement for Dooku, someone with a lot of growth potential. He's set his sights on Ventress, but no doubt she's going to end up dead, maybe not this season but by the series' end. And then he'll need to look around for another young Jedi who fits the same characteristics...
More likely she's a back up for Anakin, if he were to say take a lava bath or something
