I believe that Tobi is Obito Uchiha with Zetsu body. I think that the real Madara Uchiha found Obito and save him and told him about "the moon eye plan" after that when Madara died Obito continued with the moon eye plan and took the identity as Tobi. And I believe that Madara created Zetsu.
Your obito theory is similar to mine check it out:
Here's my theory obito's near death experience enabled him to unlock his ability to become intangible and pass through solid objects and barely makes it out alive from the underground cave with the right side of his body half crushed. Right after this he encountered Zetsu who meets this young boy with special eye powers and saves him.
I belive Zetsu is some sort of creation by madara “The way I see it is that, when madara was dying he activated the rinnegan, and let’s zetsu take the rinnengan from him, along with the harashima cells, and also some of madara's own DNA. He then tells zetsu implant all of these things into a piece of zetsu body, like a big enough chunk to make a body, hence developing a second madara. That had all his memories (or possibly he could of extracted his entire brain?) and shit.”
And later infused this body with obito who he found because he needed the body of an uchiha and this was he’s perfect chance because the uchiha’s were all still alive and he didn’t want to engage into combat.
Now what if zetsu before meeting obito implanted the rinnegan into an uzumaki child (nagato). So he could get the Sage Body |Sage Eyes sort of thing in order to study it abilities and then later when this uzumaki child was powerful enough revive madara in his prime.
But as this uzumaki got older and stronger zetsu knows he wouldn’t be able to guide and control this child anymore, which is why he needed an uchiha body to put madara’s dna, memories and harashima cells into. Which he later did when he lucky found this defenceless boy with special eye power.. Obito.