2. " No One " identity with Izuna inside of him, makes more sense to me. So far, Madara Uchiha doesn't seems like a person for entrusting his master plan to a boy like Obito.. Madara failed in keeping his clan's men intact with his brother sacrificing his eyes for him, and by making that up.. he brought him back from death when he awakened Rinnegan, told him about his master plan and Izuna has been taking care of that all these time.
a Soul transfer into Obito's body ( they both have the same blood type + he's an uchiha ) was a win-win situation for Izuna. Tobi couldn't comprehend the feeling that he got from Naruto's provocation few chapters ago.. those are signs of people who ARE HOLLOW from the inside.
Once again: the timeline doesn't make sense at all,.. of course: Kishimoto is the real Master here in manipulation and knows a way for putting all the pieces together, but right now for me: I would rather go with the idea of Izuna in Obito's body / partly zetzu, then Obito being ENTIRELY Obito.