I hear there are branches in the campaign that among other things, allow you to defect from the Commonwealth and lead to different endings. Am I right here and how do these work, in-mission decisions? Can you change branches more than once?

There are several branches in the game leading to different endings.
The branches are all handled by in-game decisions you make, and there are several decision points.

Cool. I'm playing the Defiance campaign now. Whoever did Belle's voice has an amusing accent :p makes an interesting change to have a woman heading up engineering, you don't usually see that in the space genre..

The Defiance campaign has no branching. I'd recommend playing the original campaign first if you haven't already. The Defiance Campaign assumes you followed a particular branch.
The actress who played Belle was an American, but she didn't have a southern accent. She also did one of the characters in I-War 2.
When we came up with the characters we wanted a mixed crew. Putting a woman in the engineering seat seemed like a good idea. The thing is with a game like I-War is that you need different types of people to help tell the story.
Sergei was the gung-ho hotshot who was always impatient (who secretly wanted to be the captain)
Kevin was the inexperienced crewmember who enabled us to explain things to the player by explaining it to him, without making it seem like we were giving too much exposition.
Belle was the technobabble generator, allowing us to come up with technical explanations for things you had to do in the mission.

I gathered. Meh, I wanted to try fighting for the underdog :p
Ah, makes sense.
Although the original campaign benefitted somewhat from Jefferson Clay's AI character.
For some reason Chekhov came to mind when I heard him speak..
Engineering tends to boil down to that usually ;-)
It's a shame I couldn't pay you guys directly for the game, both campaigns were excellently done and I heard too many stories of good studios falling to the wayside (Black Isle and Troika also come to mind)...