about zetsu, it's true that he is a monster, but also he is weak, he isn't that strong as the other akatsuki members, as stated by tobi himself, zetsu is not the batle type, he is the spy and sensor of the group.I don't think that the Zetsu we see is a creation from another original Zetsu. He is the monster that most Akatsuki members were thought up to be. Zetsu's ability is creating clones of himself. He even states that his clone will guide him when splitting apart to lead Taka to the Five Kage Summit.
The black Zetsu creates white Zetsu clones. We most surely see a Zetsu-like arm come off of Tobi when it was infected with Torune's bugs. Which also feeds to the theory that Zetsu can clone and create body parts that are fuseable with another Shinobi. (possibly feeding the tobi=zetsu clone theory)
IMO Zetsu didn't come around until Tobi started interfereing with the original Akatsuki that was created by Yahiko, Nagato, and Konan.
Edo Madara also states that he awakened his Rinnegan shortly before his death. Logically he wouldn't have had enough time to experiment as you state.
I'm aware of the tobi=zetsu clone theory and it is the one I favour after my own, the problem I see with that is that I just feel like madara would not put his faith into a shinobi he ran into that is capable of copying people. I feel he was purpously created. his ability's are perfect for creating an army that can take on the world to distract them. and I suspect that madara was aware that he would need someone like that, it would just be realy fortunate for madara if such a shinobi was walking around and could be manipulated into doing years of work to get madara back and enslave the world.
if we can belive tobi's story he was a motivator for the creation of akatsuki at the hands of yahiko. nagato was at that point already more then capable of using his rinnegan, so at that point in time, the real madara must have already been dead and tobi must have been around. so even in the tobi=zetsu clone theory zetsu must already have been around that time.
and about madara not having much time to experiment after awaking the rinnegan, I suppose that's true, but if you read my post you will notice that madara already perfected the dna manipulation technique on himself befor awaking the rinnegan. possibly (but can't be sure) he already created the tobi body and only needed the rinnegan's ability's to bring it to life. my theory also explains the similarity's that zetsu and tobi have, the white zetsu like arm that tobi kickes off after being infected with the bugs. the quick "instalation" of new body parts, even the quick adaptation of his body to a new set of eye's. from what I read in the manga I think it all fits