@Femme:
I don't really Get the Sage of Six Paths to be honest. I don't get the rinnegan either.
So.. the Sage of Six Paths was this dude who created ninjutsu in the past, and he had ALL THE POWERS by having the Rinnegan. He created the 9 tailed beasts from the 10-uubi inside himself.. cute lil' baby masses of power who went on rampages. One of them were given to each of the nations to balance power. He had two sons: Uchiha and Senju. Then he died.
Madara, an Uchiha descendant, being old as hell awakened the Rinnegan right before his death because he put some of Hashiramabama's cells, a Senju, into himself. Why that was a prerequisite I don't know.. because the Sage's elder son Uchiha inherited the Sage's "eyes".. amirite? So that means that only the Uchiha can awaken the Rinnegan, no?
He plucked out his own eyes and managed to, blindly, transplant them into an unsuspecting boy, Nagato, from the Uzumaki clan who had no idea he went through such extensive surgery. He's compatible because the Uzumaki are descendants of Senju…. who did not inherit the eyes of the Sage and don't use eye techniques. Okay.
Obito, an Uchiha.. a follower of Madara already, was there.. and he had half his body rebuilt from hashiramabama's cells and was just like Madara himself.. so why didn't he implant them into Obito?
Now, according to my research, only the Uchiha can awaken the Rinnegan due to their eye issues. Naruto should not be able awaken the Rinnegan to use Six Paths techniques. If not, then HOW?!
Final question: Where the hell did all of Their Rinnegans come from? http://www.mangapanda.com/naruto/565/6 And I hope it's not "Oh, those 6 Rinnegans implanted in their eyes? The one weapon that only the strongest can awaken? Makes you near invincible? I cloned Nagato's."
I do not know whether it is a good idea or not to answer this because Kishi fucked up so many things but i am still gonna try and explain how i understood it:
1.) Right, except that he split the 10-tails into 9 tailed beasts and then died, the distribution of the beasts was done by the villages themselves. The Sage never intended for the beasts to become personified weapons (now that we know their names, i think they are supposed to count as persons, ugh) and like we saw in the flashback, they were actually quite timid, despite their huge power.
2.) Now, like you said, the older brother inherited the "eyes" and the talent to use powerful Ninjitsu while the younger brother inherited the spirit and the strong body. (Now you have to shut your brain down because we all know that biology and reproduction does not work that way), Apparently, you need both attributes for the Rinnegan to awaken and the only one in the history who had both the talent and the "eyes" of the older brother + the spirit and the strong body of the younger brother was the Sage himself. You can not forget that while the Sharingan is a mutation of the Rinnegan, it can not help you awaken the Rinnegan itself without having the genes of the younger brother, namely Senju DNA. So, Madara only awakened the Rinnegan because he had both the genes of the older and the younger brother.
3.) Yes, this point is correct. Nagato could not have awakened the Rinnegan by himself because he only has the attributes of the Senju (younger brother). By transplanting the Rinnegan itself to Nagato, he probably did not need the Uchiha DNA anymore.
4.) He already had implanted them into Nagato or rather, probably let Zetsu do it because he can not leave his cave (http://www.mangareader.net/naruto/602/17), so his plan to bring love to mankind was already in motion. How Nagato did not realize at all that his eyes were exchanged is something nobody is ever gonna find out.
5.) Naruto is an Uzumaki who are in turn relatives to the Senju (if i remember correctly), so he basically should have the DNA of the younger brother, namely the spirit and the strong body. He only needs the DNA of the older brother (Uchiha) to awaken the Sharingan. I think many still believe that Itachi forcing him to eat the crow was actually, (among other things) him giving Naruto part of his DNA. Let us be honest though, Naruto awakening the Rinnegan would be equivalent to Ichigo´s Quincy power-up but it would not surprise me if it comes to that, it´s Kishi after all.
6.) It will never make sense. Even if someone comes and says that those eyes were cloned, the whole DNA thing in a manga about Ninjas is the most stupid thing to do, that already irked me back when Orochimaru first mentioned it.