I have a theory about the whole everyone changing sides deal. What if a third party comes into play as a major threat to Lelouch that could possibly upset some Britannians too. Or it could be that just many Britannians are unhappy with the way things are ran and those in power. Then, once Lelouch vi Britannia is revealed as Zero, a number of important people decide to defect from Britannia and work together with the OoTBK to destroy Britannia or the third party. I know it probably wont happen, but I think it would be better than just having random people like Kallen and the Rounds members changing sides.