Whats behind the mask is not the point. You were never meant to see it so it doesn't matter. The chest is of a gerudo in both versions though.
Not really. If all the Iron Knuckles are Gerudo its obvious Flat wasn't one. Or do we have to continue this argument now about ghosts having a gender...
No, I just checked some Youtube videos because I was curious myself. Every Iron Knuckle in OOT is a Gerudo which you can see when you break their Armour. I was always off the impression it was only Nabooru too but I guess I was wrong.