Sasuke could just turn Madara into a CT core.
He could've just done that regardless of the seals if he had it in him to do so. Nothing was preventing him from attempting to place Madara in a CT core period. Even with Naruto having the other seal, he could've instructed him to seal the core, the same way he instructed him throughout the fight.
Not sure why you think he fights better alone. Aside from Itachi, who let him win, Diedara is the only real opponent he beat solo, if you consider that Manda finish a solo effort. He would've died without the help he had in his other fights. He succeeds with help whether he wants it or not. Slim to no chance he seals Madara alone. Giving him or Naruto both seals=killing off NV.
But I ain't here to argue now just dropped my 2 cents. You got it