• Does the 1st soldier in a vehicle have to be the driver? If there’s fire on the vehicle, he’d have to be considered the driver, but when a 2nd soldier gets in, can the 2nd soldier be the driver or does he have to be Passenger 1 and then one of them has to spend an action to trade positions?
You can designate a soldier to occupy the driver or a passenger seat upon entry. Of course he's still the only one who can be hit from enemy fire and is considered the sole occupant of the vehicle.
• When you leave a vehicle, what stance are you in or do you choose one?
Since a soldier's stance inside of a vehicle is unspecified, I guess you choose one when exiting the vehicle.
• Can a vehicle drive at fast speed and then right after, a passenger get outs and runs his maximum?
I don't understand this question.