"1. The reaction of Connally when JFK is hit by the second bullet. If they're both hit by the second bullet, why is Connally so slow to react?"
It's more a question of what reaction you look for. It would not be unusual for voluntary reactions to a gunshot to be delayed. Sometimes people don't even realize they've been shot.
But in fact there is a simultaneous reaction of JFK and Connally. Watch as soon as they come out from behind the sign. JFK's elbows raise and fists go towards his neck, an immediate involuntary reaction to being shot through the neck, and Connally's hand, which is gripping his hat, flies up in front of him. He had just been hit in the wrist.
His suit jacket also bulges out at that instant when it is penetrated by a bullet.
"2. The head shot, and JFK's move backward."
The assumption people make when viewing that is that it's a reaction to a bullet. But in real life, instead of in Hollywood, bullets don't normally fling people around. Especially penetrating rifle shots that exit the body.
At the moment of the head shot Kennedy is leaning forward kind of hunched over. At the instant the bullet hits the skull from the rear the head moves forward from one frame to the next, just a bit, but very suddenly.
His head basically explodes, ejecting much of the brain matter. His torso then simply pulls itself back into the seat. The muscles still worked. He was not instantly clinically dead.