Before I begin, there are two assumptions that have to be made...
1) Madara had to use some form of splitting
2) Nagato was at some stage in his life before Hashirama and Madara fought.
3) Rinne Tensei's retrieving span is undocumented
Perhaps that's a huge part of the reason why splitting was introduced with the Muu fight - to explain the appearance of a second Madara.
This is a possible occurrence that would explain many of the events that have been happening in the manga.
During the fight between Madara and Hashirama, Madara used some form of splitting, as Hashirama killed one part of Madara, whereas the other part of Madara survived. We all know that Hashirama *won* that fight, whether or not Madara was using it to gain ahold of his DNA for a greater plan (and he did gain the DNA, seeing his zetsu project which has allowed him to recover.)
This splitting would explain why we see a young edo-Madara (http://www.mangareader.net/naruto/559/15
) that looks like he has not aged since the Madara/Hashirama battle, and an old Madara (Tobi http://www.mangareader.net/93-402-5/...apter-397.html
) -> You can see signs of aging on the side of his face. Madara may not be as old as we are led to believe, considering that the first and second hokage were a part of the same generation (since they are brothers.) That would mean that Madara is maybe 10-20 years (at most) older than Danzo.
Tobi (old Madara,) was planning on using Rinne Tensei [http://www.mangareader.net/93-458-17...pter-453.html]
to revive the part of him that was killed off by Hashirama, which explains why young Madara believed that he was brought back by Nagato's Rinne Tensei rather than Edo-Tensei - It was as if Madara knew that this was going to happen.
Thus Tobi/Madara were shocked when Kabuto found out Tobi's secret (that Tobi wasn't complete,) and revived the dead part of Madara back to the world as edo-tensei, meaning that Kabuto has complete control over edo-Madara.
Judging from the way that Tobi uses the sharingan, assuming that he's human, he has to be one of the remaining Uchihas - the other predictions, such as Sasuke's dad (who was pro-Uchiha revolution,) Obito (whose only purpose was to reveal how Kakashi got the sharingan,) or Izuna (who died after giving away his eyes.)
There's just very few options that would make sense at this point, and seeing as Tobi is using time/space techniques held by the second and fourth, it's not out of the question that he had some soul splitting ability. Based on this timeline, it's entirely possible that Nagato was alive before the battle between Hashirama and Madara.