It's really telling that you keep saying "It's really simple" but you don't actually explain it.
The soldier explains the rules, saying "Air won't pass through your lungs" and "if you touch your other self, you will be annihilated". Was she wrong, or was the movie wrong?
Protagonist goes inverted, wears an oxygen mask for the next scene, then isn't seen wearing one for the rest of the movie, yet can still breathe.
Further, wouldn't it be more effective to just take off the Protagonist's oxygen mask, rather than light him on fire, which we're already told would make him cold instead of burn him? Which the villain should have known because he was inverted himself.
And further, why does fire make an inverted person cold? Fire isn't created by igniting ice. And the fire was started by another inverted person, so it's not even like the principle of heat would be reversed.
Also, when the wife is inverted and goes back onto the yacht, and confronts her husband, why can he understand her? How is the servant on the yacht able to understand her? To them, shouldn't she be speaking backwards. And why doesn't she need an oxygen mask?
When the wife sees herself diving off the yacht, that implies from her non-inverted POV that the inverted version of herself already killed her husband. But when she sees herself, she hadn't gone inverted yet, so that initiates the paradox of how she could see herself doing something she hasn't done yet, and why is she not caught in a loop from the point she went inverted? It seem like her life-cycle would always end at her going inverted to kill her husband.
