like someone said before, itachi told sasuke that he met madara wandering around the forest in that time. and that he helped him to slaughter the clan. so why would he lie to sasuke about that?
furthermore, how would itachi got his ms if he didn't kill shisui? faking to kill shisui just that he could slip into madara's role and THEN murder the clan just doesn't make any sense. to much of a fuss though.
theoretically he coud have got it like kakashi did, but then he wouldn't have told sasuke to kill his best friend to get it...
so what i was trying to say is that madara most likely is not shisui. and i just don't have the desire to be the 1000th to explain why obito can't be madara...