I highly doubt Tobi is Izuna for one simple reason and this reason is small things people don't notice. When ever something said is true they will show a scene of it in a flash back(not in all cases obviously). Even if the truth was altered abit. Kinda how Sasuke seeing Itachi killing his own parents and the whole tears running down his cheek thing. In this case, when Tobi tells the story on how Izuna died and it shows a flash back of him dead in a coffin with his eyes covered. This proves that he did indeed die but whether Tobi told the truth on how he died remains hidden. I guess the real Madara has to shed the light on this.
