GNW Madara (Obito at the time) is the aftermath of Obito retrieving Madara's Rinnegan from Nagato after Nagato passed away. Following Obito's ac*ulation of the Rinnegan, he changes his mask. Your Zetsu claim is partially correct in that following Obito's childhood injury, Madara had already been making white Zetsu clones from cultivating Hashirama cells. To help heal Obito, Madara cultivated Obito with Hashirama's cells as well. This is why Obito c*e his eye's Kamui at a much faster rate than his counterpart in Kakashi, as the chakra supply isn't being continuously resupplied like one's supply would if they had Hashirama's cells in them.