Interesting. That's a completely valid idea. The speculation in the beginning is a slight jump but the rest is very solid and I think this could actually happen. This is more likely than the Elder Son theory anyway. + repMadara (591) says "As long as you know the seal, you can release Edo Tensei's summoning contract itself. There's nothing more troublesome than doing that an undying body....unlimited chakra...it cannot be controlled."
Doesn't this sound like a description of Tobi? As in, this has happened before, someone (Tobi) knew the seals of ET and released himself (whoever that was).
Tobi has an undying body. He seems to have unlimited chakra. He can't be controlled.
Tobirama and Orochimaru are the two possible candidates for 'carelessly using a forbidden jutsu' ET. Who (who Tobi is/was) they resurrected is still in the dark, but a flashback of Orochimaru bringing back 'Tobi' combining the 1sts cells with his and losing control of him as he released ET would explain a lot. Like how he can take a Rasengen in the back by Minato or survive the paper bombs of Konan or detach his infected arm and replace it vs Danzo's henchmen.
Combine this with 'They were my eyes to begin with' one could say that Izuna's eyes were given to Madara who obtained the Rinnegan, thus using Izuna's eyes for EMS and eventually Rinnegan. It would be true that those eyes, somehow implanted in Nagato, would have been Tobi/Izuna's eyes to begin with.
Just some thoughts. Not saying its proof. Speculation involved.