Im not sure that nagato was given his Rinnengan i think that he had it naturally and that he was found and trained by tobi or madara after J man left them because he had it when his parents were killed and killed the men who killed them with it if you remember. It is in Chapter 444 the answer page 14-17.
Nagato is an Uzumaki, which means he's descended from the Senju, not from the Uchiha. Him awakening the Rinnegan without any help at all, makes about as much sense as Naruto awakening it without any help at all.
Nagato's story of how he got the Rinnegan is exactly that - a story. Everything he remembers could have been part of a genjutsu put on him by Tobi, designed to turn him into who he became.
Kishi hasn't exactly explained 'Nagato has Rinnegan' very well to begin with, so just awakening it, being of the wrong blood line and heritage, is very suspect at best.
Besides, Tobi himself said that he gave it to him.
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Aside from Susanoo (how does that fit in there), that theory on The Creation of All Things jutsu actually makes alot of sense.
In my earlier post a couple pages back, I talked about how Susanoo seems to be an extension of MS, EMS, and Rinnegan, which behaves as a conduit for other techniques. When you see a ninja with a sword, he's just a ninja with a sword, and it's power is of a ninja with a sword. When you see Madara with Susanoo active, wielding a sword, it's a giant ninja with a telephone pole for a sword, and has the power of such.
Then you see when Sasuke uses Susanoo to manipulate Amaterasu into a shield. And when Itachi uses Susanoo to manipulate the Sword of Totsuka and the Yata Mirror, both metaphysical objects. And then when Madara uses Susanoo to form different hand seals simultaneously, summoning a giant meteor.
It seems obvious to me that Susanoo can do whatever the user can imagine and has the power to do. What if it was the tool that the So6P used to manipulate and shape life, using wood release and the Rinnegan.
There are currently only three known ways to revive the dead. Chiyo's and Tobirama's were formed after Madara's death and unless he developed the Rinnegan around the time of Izuna's death, he probably didn't revive him with the Outer Path, cause Nagato couldn't revive anyone he killed beyond his time in Konoha.
The time when Chiyo and Tobirama created their techniques is actually up for debate. Tobirama was alive when Madara was alive. I'm pretty sure Madara was fully aware of Edo Tensei before he died, since he knows all about the technique upon being revived with it.
Whether or not he actually used Edo Tensei is probably not the case. At this point, I remain convinced that
someone revived Izuna at some point. It could have been Madara with his dying breath.
It could have been whomever Madara was allied with at the time of his death (cuz I am sure he wasn't acting alone).