I'm pretty sure that his mods are not, in fact, based off mutations. The only people we've seen walking around with air pipes in their arms have had them put there by Orochimaru. Same with the stretchy arms and what not.
I mean, fuinjutsu might be technically involved in the body-hopping (we dont' see any fuinjutsu script though), but I'm not sure how that detracts from the point that he invented it. With his Learning skill.
As for True Immortality, Orochimaru claims he has it during his fight with Sasuke after Sasuke tried to kill him while he was weak. It's when Orochimaru reveals his true form, as a result of his extensive body modifications.
Plus, he spent quite a while in the company of two other immortals (Hidan and Kakuzu), and didn't seem that interested in their bodies or how they worked, so it would be rather wierd for him to overlook such a thing if he didn't believe he had better.
While he didn't invent them, he has a far better track record with them.
It presumably is more indepth than JUST a genjutsu, to explain why the Uchiha don't give every single member Mangekyo Sharingan.
Kabuto's feats are still impressive, but just not as much so. But even then, my point was that Orochimaru still deserved a lot of credit for everything that Kabuto achieved. Orochimaru made a ton of shit.