I also like the idea Marianne staged her own death. Either she planned the whole thing, or she knew about the cult and that they were going to kill her. Being as CC has been watching Lelouch since at least the age of 9, and that CC keeps up a correspondence with Marianne, I would suspect she has a connection to the cult. If VV replaced CC as leader around the same time, that could all tie in? CC took Marianne's side, VV took Charles'?

The fun of speculations~