Why is it not "mężczyżnami"?
Actually, I've just checked, and the correct instrumental plural form is mężczyznami.
Would "y'all are" be an appropriate translation of "jesteście"?
Yes, if it helps you distinguish between "ty" and "wy", you can use it. It should be accepted everywhere for the plural 'you'. I hope.
Is saying "Wy jesteście" like saying "you all are" or "y'all are" (in southern us term). Because English doesn't have cases like in polish so we add "all" in front of "you"