And how might shooting the driver save them?
By reducing the vehicle's speed possibly or causing the driver to lose control of the vehicle and, ideally bring the vehicle to a stop by running into an obstruction if even for a moment. The last thing you would want is for the vehicle to retain or increase its momentum in an unobstructed avenue of attack against the crowd so anything you could do to decrease his momentum and/or impede his plan of attack would give the crowd time to get out of the way. Every second you could delay his forward progress would save lives. 30 seconds would just about completely mitigate the attack I would think.
I most seriously doubt it
Section 2901.05 | Burden of proof - reasonable doubt - self-defense.
(B)(1) A person is allowed to act in self-defense, defense of another, or defense of that person's residence. If, at the trial of a person who is accused of an offense that involved the person's use of force against another, there is evidence presented that tends to support that the accused person used the force in self-defense, defense of another, or defense of that person's residence, the prosecution must prove beyond a reasonable doubt that the accused person did not use the force in self-defense, defense of another, or defense of that person's residence, as the case may be.
You're clearly using deadly force in defense of multiple others here. Unless you really screwed it up, you wouldn't even go to court. You would have to exercise the same level of caution that you would have to exercise in any self defense shooting.
Not much. The time to think about it is now.
Well, we are thinking about it now. In the Army, we had "battle drills" that were designed to reduce thinking e.g. react to near ambush, react to far ambush, react to contact etc. Battle drills are great for a lot of commonly encountered scenarios but when you're encountering something as uncommon as the Nice truck attack, there is no battle drill. You're almost certainly gonna have to assess, think, plan and execute so the best way to train for something like this, IMO, would be to train yourself to assess, think, plan and execute while your heart is pumping 120+ bpm, while you're breathing 30 respirations per minute or more, while your vision is narrowed and perception of time is dilated and you have adrenaline running through your veins.