I don't think there's any chance of Kogami being Kamui. Kogami wasn't ever crazy. He was one of the more sane individuals, aside from the stupid Sybil's all too error-prone evaluation. Sanity is just statistics in this show anyway, as it's up to a computer to decide who is sane and who needs treatment. I have no such recollection of Kogami that he would be willing to kill random people to further his goals. Would Akane even have fallen for some senseless murderer?