What if the Arthur Roemig of the NTC Trinity never defected to the Neo Terran Front?
If Arthur Roemig never defected to the NTF Bosch would have sent another ship. That ship could have failed and the Shivans may never have reached Capella.
What if the Sathanas fleet never made it to the Capella star?
I doubt that would ever happen, unless the GTVA managed to effectively close down the Knossos or nuke the Gamma Draconis node. If that happened I expect the Shivans would have gone back to go somewhere else or find an alternative route.
Would the Shivans have retreated if the Capella sun went supernova before their Sathanas fleet could do something with their subspace drives to find another way back home?
I think they knew what they were doing and they weren't stupid enough to make that mistake. The Sathanes which did not jump either sacrificed themselves for the other Sathanes or did not have enough energy left to make the jump (you can see they look 'dead' since they lose their red glow).
In one of the Bosch monologues, you can see clearly the Capella star is building up it's supernova stages - is it that the Capella star is going supernova on it's own - or that the Shivan presence in the Gamma Draconis magically makes the star react so badly . . .
Like Dark Hunter, I doubt it was the Capella star in that monologue. I doubt as well that it was a black hole. It may have been Sirius A and B which is a binary system, but that may have been under GTVA control at the time, I don't remember.
Another thing: Capella in FS is a yellow star similar to Sol. Such stars do not, ever, go supernova. They aren't large enough. The Capella Supernova is therefore artifcial in nature, not natural at all.
IMO, the Capella in FS wasn't the Capella in real life. Another star in real-life was renamed to 'Capella II' for some reason and then people started informally calling it 'Capella' until it was actually known collectively, as Capella.