1. His Rinnegan does contain the power of two and Naruto's Six Paths Sage Mode is comparable to a ten-tails hosts', as confirmed in the Databook. I saw you already try to deny it by saying that it was mentioned by Madara that one needs two Rinnegan for the full power, but you're wrong in your reference. It is not that one needs two Rinnegan, but that one needs their two original eyes. In Sasuke's case, both of his eyes are his and thus he can use them to their full power.
2. Neither Itachi nor Shisui have Perfect Susano'o. That's in a game, regardless of who made the designs. The same goes for your size comparisons, though Madera's four other Susano'o are clearly the same size as Sasuke's. Denying this is just sad.
The other points you make are just mere possibilities that could have occurred. No point in trying to argue against them.