<Campaign_Set> Sandbox_Empire_at_War </Campaign_Set>
this is just a reference as to what campaign the faction setup belongs to. I dont know if you can remove the rebels or imps. it may not be allowed.
This may be a pain, but the first thing i would do is find the answer to this first. I would take a clean copy of the game, and change the name of the imps or rebs, and nothing else. see if it will work. if it does, then you probably have some sort of syntax error. if not, then its probably hardcoded.