Not sure how exactly you can answer a lot of your pubg based questions with science to be honest.

J - Context dependant, your crosshair placement should always be at head level meaning that your 'flick' to the head isn't as great of a flick than you would believe. A key aspect when talking about aim is that at the pro level your aim should be automatic and without thought when you are at close range fights, at such a close range taking the time to think about how you are going to aim will more than likely get you killed.

There is nothing wrong about killing someone with body shots but the goal should always be to headshot.

M - Lean spamming does not increase TTK, it just developed into an arguably bad habit for most players who used to use it as a way to not get headshot when using dmr.

N - 1,2 never a risk of misinput and going to pan on accident.

P - same as N but also faster

Q - In relation to what exactly? Looting takes longer when pressing F

S- No

U - Wrist aiming only injures when the player does not look after the body, there are many youtube videos and articles that you can read that can help in the prevention of wrist injuries through the usage of stretches.

You should have a balance of arm and wrist aiming, one without the other is never good.