Unanswered questions
How did the Sleeper ships get to Sirius in the first place? Did they take jump holes or is there some other means of traveling from system to system (maybe very large ships can jump unaided, Babylon 5 style. That would explain how the Osiris reaches otherwise unreachable systems during the plot) If they took jump holes then surely the route back to Earth would be traceable (unless the holes subsequently collapsed)
Oh and this get's me thinking, In 800 years how come the Coallition has never found the colonies? Maybe they found somthing else (i.e. the Nomads) first?