Perhaps you could adapt Dirty Secrets for a good chunk of what you need. It's a game where one person is the PC, while the rest of the group plays the part of the GM. My only experience with the game was on Story Shtick, but it sounded like a lot of fun.
I'm not sure how combat works in the system, but considering that the vast majority of combat is heavily, heavily weighted in Ezio's (or Altair's) favor anyway, that may not necessarily be a concern. The sneaky-sneaky could also be tricky, but without knowing the system, I really can't comment on that.