1. Get someone to unseal Haku, then grab Hinata or Neji to do a byakugan gender examination.
2. Get some one to unseal Haku, restrain Haku by the Raikage and let Kakashi lift up Haku's skirt.
OT: I actually thought there was no way someone could argue that haku's a girl but I read about 90% of it and now after reading your theories I think there is a big chance he/she was a girl.. you came up with good points..
Except the index finger/ marriage finger thing, I'm a male and my index is longer then my marriage finger..lol