Clearly its been given because of a combination of 1, 2, and 3, and was given in real-time because Oliver thought he handled it deliberately, but with the benefit of replays I really don't think what Pogba actually does supports that interpretation. In fact, I think he quite clearly loses the ball turns around, sees it again, misses his header and the ball strikes his hand which is in a fairly normal position for someone attempting to head the ball like he did.
I agree. I think the replay shows 100% that his absolute intention is to head the ball, clumsily and stupidly with his eyes closed. Undeniable idiocy, but I can't see any intention of him moving his hands towards the ball purposefully, or putting them in a position which vastly increases the chances of it deflecting the ball to his advantage.
I understand why Oliver has given it, but I feel certain that if referees had the opportunity to review incidents via video replays, the penalty would not be given, as upon seeing the replay there really is no visible sign of any deliberate handling of the ball. His eyes are closed, he gets caught in a panic to regain position, he leaps, and then he thrusts his head towards the ball. He gets it all horribly wrong, but he still doesn't deliberately handle the ball.
You're right that the law is far too vague and leaves a great deal up to the interpretation of the referee. You see some harsh handball calls given all over the pitch, and some clear deliberate handball incidents not given. It's a different interpretation of the rule, game by game.
I've often thought handball in the penalty area should be an indirect free kick, similar to a pass back. Unless it prevents a clear goal scoring opportunity, like being saved on the line like Suarez did a few years ago. Otherwise, it's a very harsh punishment for a call that is guesswork a lot of the time.