my theary is that tobi is indeed Izuna,wen Madara took Izuna's eyes,i thik that Madara gave Izuna his...
Remember that even though Itachi sight was almost gone,wen Sasuke put them in,there was a new light
so if Madara had became blind from spaming his mangekure,if Izuna puts in Madara's eyes...a new light shud come to play also...
so i think they swap (exchange) eyes,and they both got EMS...
and Izuna was probably afraid of his brother,so he couldn't do wat he wanted all along,so after Madara's death he decide that it's his time to shine,and uses his brothers name because it strikes more fear into people than his...,because he always fear'd his brother,wen kabuto show'd him the 6th coffin,the fear came ova him again...thats my theary..haters can bash and flame until thy kindom come,i don't give one s**t