Guns are lethal force. If you shoot someone, you shoot to kill. That’s it.
You don’t shoot people in the leg. Even a shot to the leg has a very high chance of causing a fatal wound. Nick the femoral artery and the guy will bleed out very quickly.
Hardly. I wouldn't even count on a leg shot from a pistol to immobilize the attacker. Even rifle shots to the pelvis are not reliable to stop an attacker.
But I agree with your larger point. Once the decision to use lethal force has been made, that force will be used until the threat is ended.