The Mercenaries use machetes and knives because of the Electromagnetic properties of the island interfering with their weapons. In the opening sequence of the film, however, Inara's father had no problem using a pistol to shoot, and kill, the other soldier while on that same island.
The women of the Island, although supposedly all from the same tribe, obviously came from several different backgrounds
Although the tribes of the Amazon historically have pierced their noses and lips in such a way as to create the appearance of having whiskers, they did not very often have a single belly button piercing.
The opening credits makes reference to the "Band of Viligantes" as opposed to the Band of Vigilantes.
During the fight with the mercenaries in the alley, Inara is so intoxicated that she can barely stand, yet at the same time sober enough to defeat three highly trained soldiers in the fight.