Who's pluralizing?
I meant pluralizing the possessive (no idea if that's actually the right grammatical term) part - the נו in רבינו, but when I think about it my question has nothing to do with the plural part, just the possessive part. You explained that רבי is ראש בני ישראל. But if that's the case, why would you ever add נו, making it ראש בני ישראל שלנו - רבינו. If he's ראש בני ישראל, and you're part of בני ישראל, isn't the שלנו superfluous?
ETA: Also, if you're correct, why isn't it רב"ינו? Or is it sometimes written that way?