I have a theory on how C.C. will regain her memories.
Rather than making another CG thread, I'll use one about C.C.
C.C. will somehow end up in mortal peril (it does seem that she can die now). Realizing that yet another person that is important to him is about to be taken away, Lelouch yells out C.C.'s real name in the loving/longing/caring way that she really wanted and the shock will jolt her back to her normal self.