Are we discussing the outcome of this suggestion already? Like I said, it's going to be interesting.
Here is the other problem with your scenario. Galen wasn't interested in bumping off Vader until the end of TFU...
So Galen isn't likely to be trying to bump off Vader and Caedus gets an apprentice that will jump ship at first opportunity or cut Caedus' head off at first opportunity.
The absolute best I can possibly for Caedus is a draw where he and Vader kill each other. If we want to talk about who is the most likely to survive that kind of a Kaggath:
- Ben Skywalker - let's face it, Vader wouldn't be interested in killing his Grandson, particularly when the kid would willingly join forces just to take down the guy that murdered his mom.
- Darth Vader - If he gets Ben Skywalker to join him, Caedus is looking at an intelligence fiasco of the highest level, considering Ben's memory. While Caedus' is also a grandson, fact of the matter is he bumped off Luke Skywalker's wife (something that has to be thrown into this Kaggath, because it was how Caedus became a Sith Lord), possibly tortured Ben, and took pot-shots at Leia and Jaina. In short, Darth Vader is going into this thing royally pissed off. Part of the darkside is rage right, well Vader will probably have Jacen beat in that department.
- Galen Marek - I don't see him betraying Vader. He'd be curious about Ben to an extent, but Ben Skywalker would also be a valuable intelligence resource in taking down Caedus, so I don't see a confrontation there. I'm also not sure how much Vader bought into the rule of two thing.