Regarding that 'one eye'...
I just had a crazy thought, what if the reason Madara only had one eye was because Izuna had the other one? Isn't it strange that he only took one sharingan eye, I mean, how many single-eyed uchihas are there? Why wouldn't he take both eyes if he had the opportunity?
Madara had his sharingan-eye on the left side while Tobi has (and always had) it on the right side, hence he can't be the long haired Tobi. Izuna on the other hand, under the assumption that Madara gave the other eye to him, would (or should? dunno how the eye transplantation-technique actually works) have his eye on the same side as Obito, therefore it's possible that he's the long haired tobi! Well, what do you think?
I humbly apologize if anyone has mentioned this before. Personally I don't actually believe that the long haired Tobi=Izuna, I think that would just be too complicated. However there is a possibility (a reaaally small one though) that Izuna is if he was still alive at the time.