There are many theories but he is a clone. think about it who else knew madara's plan noone just madara himself. he is a clone of the real madara. madara problaby made him to accomplished his plan and then revive him to rule the world.
That's true too. The hole is on the right side of his face so he could be Obito and he took the sharingan from Danzo after Sasuke killed him. He didn't have the sharingan there on his left side.