My theory is that the current tobi is just a more advanced zetsu with sharingon / rinnegan. Madara knew he was dying and assigned his best zetsu to take over. Madara's plan was to have Tobi assume his identity and implant Nagato with the rinnegan which he commanded Tobi to manipulate Nagato into resurrecting him when the time is right. That's why he told Mu that it finally happened and came to the conclusion that Nagato grew believing his plan went as expected.