1 Body of the car will always bend since every metal will strain under stress (when force apply). So matter how you drive, force will bend the car, but by how much is depending on how large is the force, applying to the body of the car, as well as which direction.
Car manufacturers will try to make cars as stiff as possible, but it always a compromise between stiffness and weight. You could build a car to be very stiff but would be very heavy, ie. a tank. Or you could build a car very light, but with little stiffness that it drive like a boat.
2. Depends on the car design.
3. Adding a strut bar will stiffen the chasis of the car. By how much, depends on the design of the strut bar. But yes, it will stiffen up the chassis.
4-5 to measure the stiffness of certain part, they use strain gauge to measure the force at local point. But they normally use Finite Element Analysis to calculate the stress of the complete chassis. Also, they use test rig to measure the torsion stiffness of the car, by applying moment to the car and using dial guage to measure how much the car twist.
