Madara said if he used the Rinnegan, not that he would, and even if he did mean it with one eye, that full power he referred to is Limbo. He didn't use anything else.
He used Gedo mazou nine dragon to subdue all nine beast in seconds.
He wasn't talking about just limbo, the outer path is the strongest path and utilizing the Gedo path to its fullest extent is part of using the full power of the rinnegan.
While the Rinnegan operates in the same way, Madara only was able to use it because he awakened them. And yes, Tsukuyomi could be used like that, unless you think you magically forget what you learn after being released.
Madara awakened them, and yes without the eyes utilize a few techniques.
Madara's will contain his chakra in them, so anyone infused with it also gets the same access.
The tsukyomi assumption that you put there would be pointless, considering that he would magically loose this so called "tsukyomi rinnegan."
It would completely defeat the purpose because he doesn't have the rinnegan and Nagato does, why bother teach Obito if he can't use it?
Answer is that he can, but in a different method.
Madara also only said to use it after Obito learned how to use the Rinnegan, and we never seen him use it, so you don't know when or how he practiced. I never said Nagato had six Rinnegan, as I'm not stupid. I said the chakra transmitted allowed them to use it's power.
The chakra transmitted from the rods allowed Obito to use it's power too, despite with no eyes.
Like I posted before, we haven't seen Hagoromoo use it either, but we won't deny him his rights of usage too.
Obito with the rinnegan showed techniques against rikudou Madara anyway.
Madara never said to use it after he gets/use the rinnegan.
He stated to use the black rod when preforming the techniques, since Nagato has the rinnegan.