Here's what I think
What we do know about Tobi:
he was alive when Madara was alive
when Madara was summoned he knew him
Madara supposedly died atleast 60 years before Nagato was born, yet somehow the first thing EdoMadara said was so that brat Nagato..
this tell me that either Madara died after Nagato was born, so he lived a lot longer and died not much than 30 years ago, not like 80 as we thought..
And we is it so hard to believe that Madara and Izuna just switched their eyes and both got the EMS???
No one said that only one brother can unlock the ems..
as to how can it be Izuna and be so old:
he has some DNA from the 1st Hokage(who was the only one said to be as strong as Madara) + the EMS is said to prolong life + he could be using some jutsu like Orochimaru to stay alive younger(switch bodies anyone??)
AND for the Final one Theory to who might Madara Be:
Okay bare with me for a minute and keep reading
when Madara fought the 1st, he stole DNA from him,
It was said that Zetsu was created from the 1st hokage's DNA,
Zetsu it the most loyal and beside Tobi from the beginning(Tobi trusts him uncoditionaly),
Zetsu has the power to transform perfectly as someone else, even copy their chacra perfectly,
So what if Madara created Zetsu (who else would be?), Since Izanagi is said to have the power to alter reality and create things AND the power of the Senju - to bring it to life (In a chapter it was said about the two sons of the Sage of six paths, that the son with the Sharingan could create and the other one could bring the creation to life..), and that's why it has the 1st DNA - to bring it to life...
and there is a posibility that Madara before dying casted a genjutsu to implant his thoughts into a Zetsu ( the way shi's eyes casted a strong enough jutsu to free Itachi..), and Zetsu just copied his chackra an outer appearance
+ before Sasuke fought Danzo, Tobi just ripped his hand of and it looked like white putty..
And Tobi said that he is just a shell of his former self.. couse he has the memories and thought of Madara but not the power of his body
+ Tobi said sth like: I'm nobody.. It doesn't matter who I am..