To start things off, we need to all understand Madara and the FACTS we know about him.
1. He fought the first Hokage - Agreeable.
2. He unlocked the Rinnegan through this fight, or shortly before his death - Agreeable
3. He is in fact dead - Agreeable or he wouldn't be able to be revived, assuming we take things at face value.
4. He apparently has no connection to Gedo, and I say this on the basis of Nagato. After continual use of Gedo, we see what Syncing with it has done to his body.
This leads us to believe that there is not ONE, not TWO, BUT THREE sets of known Rinnegan, Madara, the SO6P, and however Tobi has acquired his. We know Nagato got his from Tobi.(Counting these as one pair). Simply put, Madara and the SO6P are pretty much some of the strongest people, or very high in that ranking thus known on an individual basis; so Tobi needs to also be someone on this level as well so it makes logical sense.
First off what's agreeable? Synonyms: pleasant - nice - pleasing - enjoyable - likable - sweet . So you are saying - All these incidents were enjoyable to you and that's relevant somehow.. :flaw:
Now we come to your facts: I don't see any indication of three or more sets of rinnegan so far as . It appears there were only two sets - So6P and then Nagato's. I don't agree with your assessment that Tobi has a third independent set of it other than Madara/Nagato and So6P's.
Having a third set means Nagato's rinnegan was his own. If Nagato's was transplanted, in that case everything seem to imply that it was Madara's eye he got, by whatever means.
If Tobi transplanted it - for he claimed that he gave it to Nagato- then he would do so only if he was incapable of developing it himself for some reason- like not having the right body/chakra/capacity for it. Tobi is all for power ups- he is more like Kabuto than Nagato or other nins who get powers without popiing pills all the time or injecting alien DNA to themselves.
There is still a chance that when he said he gave Nagato rinnegan he meant he used genjutsu on some Konoha shinobies to kills his parents to awaken it- that will make your theory of three rinnegan sets correct. On the other hand if Madara stole his from S06P's dead body we will lose even the second set from out equation..

(JK)
I find this very important, because Tobi talks as if he knows either the "Destiny' or "Fate" of the full bloodline of the Uchiha, meaning he has been around for a while, or somehow knows things we do not, which is quite obvious. But again if we take this at face value that already eliminates half the people many suspect Tobi to be, it's that simple.
These facts are then the result:
1. Madara fought the 1st Hokage - He lost - He unlocked the Rinnegan
2. Sasuke is "Fated" or it is his "Destiny" to fight Naruto - But for what purpose in Tobi's mind? Well, now we know how the Rinnegan is created, so it would be reasonable to say that it is for that very idea.
3. If we use Madara's fight vs the 1st Hokage, AND Naruto vs Sasuke's "fated" fight things become a little more predictable and logical.
We can infer that either the Eldest son, in whatever possible way, knew how to unlock the Rinnegan, and thus fought the younger son for said purpose. Or he happened to come across it by chance; regardless, if things are following the same pattern of logic it would be inferred that the eldest son DID IN FACT unlock the Rinnegan through that fight, and either died soon after or was put into an incredibly weakened state and I'll explain this specific example in a second.
Manga gave indication that Madara was not alone and working with someone. He was also alive after the battle in the Valley of the End and died in some other battle just after reviving the rinnegan- As he knew Nagato.
He and Izuna had been consantly trying to decipher the tablets in Uchiha hide out. He would share the info with his partner who most probably is an Uchiha, seeing the effect of shaingan on Kakashi- only known non-Uchiha sharingan weilder. You are making too much a stretch when you assume that it can be only Elder son- for all we know he is as dead as So6P himself.
Son of the So6P, if alive, on the other hand would/should feel insulted if he had to go hiding behind Madara's name at any point of time. Even more so, of plotting against Uchiha's by Kyuubi attack and then wiping out his own clan. So your conclusion about that since he knew the history he must be the elder son doesn't fit.
This is where, for me, it delves into really paving out some in depth detail. So what we have concluded from this is that
Madara, and SO6P both had Rinnegan. Nagato did also, however, there is an outlier: Gedo, we don't know where it has yet come from, if there is more than one, if it is reproducible, or what it's full potential is. However the chapter 447 is VERY important for a few reasons, first let us think of how the Sharingan progresses. Do you know? It's all emotionally based to be pretty blunt about it. How to you unlock it in the first place, how does it advance to the third tomoe, how do you unlock MS? All are tied to emotion. We KNOW for a FACT the Rinnegan is the Sharingan, so UNLOCKING Gedo is a HUGE possibility, and purely logical when we simply look at how Nagato used it.
Yes we do. Was being this wordy necessary?
GEdo
Moreover, what does it do? Well with the Rinnegan the controller is quoted as being able to control "Life AND Death" and perhaps this can only be done through a syncing with Gedo. Gedo may allow simultaneous, or linked use to the many paths Nagato apparently did not have before this point, and as we see Gedo is also using similar abilities to the outer path in this chapter, so there should be a reasonable connection here. Moreover, the black rods connected to the back of Nagato look very similar to what are being used for the other paths/bodies so perhaps another connection via Gedo - But moving along with the main point - the more people attempted to be revived the less of the consumed souls it can return based on a time basis.
Manga implied the criteria of time.. where did you deduce about the number of souls...
According to Jiraiya Nagato could use all the elements and jutsu's since the age of ten. He may not have been as effecient at that age but he was capable. He summoned Gedo mezo after he lost his legs...
Death is a very interesting factor for me, and now thinking about it "Or" would be the correct way for it to be used in English if it was able to do "both". We know it cannot be used for "both" seeing as Madara had noted the ability was supposed to be used for him, thus its affects are only producible once.
Because of this information we HAVE to conclude that Tobi HAS to know how Gedo works, and why he does not abuse it like Nagato did after syncing with it. It would be reasonable to say that Tobi is attempting to find a way around the negative affects Gedo has on the user(This is really key considering Itachi always mentioning everything has a negative side), thus, he has used it, synced with it before and has some sort of work around for it.
Tobi is always trying to find a way to get others to do things for him- he didn't sync with Gedo- didn't he wanted Sasuke to replace Nagato?
Which leads into the next area of what the hell is Tobi's real objective with Sasuke?
It was said after Nagato's failure that Tobi would look for someone else to sync with Gedo so that the "ability" Nagato used could be used again for Tobi. But all users thus far have, in some way, had possession of a Rinnegan. So, the whole objective of Sasuke vs Naruto is for Sasuke to unlock the Rinnegan for the eventual syncing of Gedo, it only makes sense.
You know so far this is the most interesting Idea- Tobi trying to revive Sasuke's rinnegan via his fight with Naruto... But would Sasuke also need some Uzumaki to develop it or something senju to use it to full extent?
With all of this information we're faced with very limited options moving forward when taking in this information: Tobi has to know about the past, he has a Rinnegan pair, he knows how the Rinnegan works, how Gedo works, wants Gedo to be used FOR him, and has some some eventual plan. [/color]
Conclusion
And thus this leads me to my prediction: If Gedo can only be used once for the "Life and Death" ability, and Tobi knows this then it would be reasonable to conclude someone who has unlocked it through an emotional fight; Tobi almost has to be the Eldest Son of the SO6P. He unlocked the Rinnegan in his fight against the younger son. Think about this, he has always had access to being around the younger son. If in fact the younger son was able to use the "wood tech" this would promptly explain where, how, and why Tobi has Zetsu. Moreover, if you unlock the Rinnegan before you "dye" or are in the process of "dying" then Zetsu would simply negate this. Madara did not have Zetsu, thus in his battle he died - why he is able to be revived. Sasuke will be aided by Tobi, thus able to unlock the Rinnegan explaining Tobi's need for Sasuke. Nagato had his implanted so that explains why he was alive.
The only questions I have left is how the eldest son had Zetsu in preparation for all of this? My guess is, he didn't. In fact, I believe this is where the "shell of a former self" and the "Life and Death" ability come into play. The Eldest son was, as we know, defeated but did not have zetsu around, if in a bind like this he may have used Gedo to steal, and store his own soul. Zetsu, as we know is created but has a conscience/personality, thus leading to the explanation of why many of us think Tobi is using a Zetsu body (Being able to recreate arms, yet still use his eye techs etc). Regardless of this small part being wrong or right, it would make sense that if he stored his soul in Gedo and another Rinnegan user synced with Gedo then used the ability to return that soul to Tobi, how the Eldest son could have been preserved for so long. And from this, everything stated above can be explained as to why he knows it, and simply justifies all actions he has taken, what he plans to do, and why he talks so much about destiny and fate as he has been around for so long.
The repetition and history throughout the manga has helped explain many events, and it seems reasonable this is one of them. This really only comes as the logical answer to me, at least at the moment, and hopefully it makes sense to everyone else ^_^
Discuss~?
Your theory makes me believe it was Izuna as well or any other Uchiha.
Really , You have posted nothing which would compless us to consider that it must be the elder son of the sage.