not trying to destroy your theory/hopes , but we dont have enough info on kagami for it to "make complete sense" we can only geuss/assume everything around his existence in the manga. but i still think its madara(or atleast someone very close to him) . look at the way itachi addresses or talks about madara, like he truly knows him and his being, he did meet up with him and got him to help slaughter the uchiha WITH HIM, and we all know itachi is smart and their is no way he can be tricked either. and i dont think its a coincident minato and jeriyah had the same conclusion. jiryah said it himself that he was the only one capable of controling the kyubi when naruto was born, they could have all make the more reasonable assumption of geussing it to be someone who is still alive but no, its madara! im not gonna lie, this isnt even 100 percent and their are still plot holes blah blah blah![]()
As much as some of the evidence shows it not to be Madara behind the mask, the recent interactions with Itachi make me believe that it is infact Madara if anyone were to know other wise it would be Itachi yet he still refers to him as Madara and made no effort to give Naruto any further information on him.
Just as Itachi, Madara/Tobi truly is a shinobi surrounded by darkness.