1. 'Rank S missions' is only ever talked about in fanmade comics like the one where everyones stats are shown, their age, the age they became chuunin/jounin. AFAIK officially there is no Rank S... unless the comic is actually official data.

2. Zabuza was never Rank S. If Kakashi can kill him, and Itachi/Kisame/Orochimaru are 'Rank S missing-nins' and can kill Kakashi... I don't think Zabuza was Rank S

3. Kakashi said it was B rank, with the possibility that it was A rank when the Demon Brothers fought them.