I bet kakashi wakes up to see long hair obito with mask on and he leaves withs runs body, and wonders who it is for the rest of his life. That's way kakashi wasn't surprised when he called him kakashi and why kakashi would have believed he could have been madara.
Makes things more dramatic.
Instead of orochimaru the guy who ordered it, it was madara. Remember madara was head of the mist village when kisame lived there? Guess what that's this time period. The persuaded obito to follow the plan. And this is why kisame called him mizukage.