Owned kenpachi? You must be joking. Zaraki is making sport of him most of the fight, besides his bells and eye patch. And then, the only reason ichigo overcame kenpachi was only illustrate the bond that a shinigami has with their zanpakuto.
As for yachiru theory, that's quite interesting. He says his sword is always in shikai because of the force of his reiatsu, but who's to say this isn't also the case with materialising his bankai?