ALL damage to a car is repairable.
The real question is can it be done cheaply enough to make it worth whyle and can it be done well enough to be as servicible as before the damage.
Undercarriage damage usually refers to the wheels, suspension and axles. There can be a variety of damage to all of these parts...literally hundreds of possibilities from flat tires and damaged wheels to broken shocks, springs, steering mechanism and axles.
"Reversible" means something you can take back, so it basically means damage you can repair. For example: You could spill something on a car seat and have it cleaned, the same way you can replace a broken headlight.
If parts of your undercarriage scrape on the speed bumps as you pass over them, yes, they could do damage to your vehicle.
undercarriage
You do not need the plastic cover on the undercarriage of the Beetle necessarily. It is designed to protect the undercarriage from road debris but it is not necessary to the function of the car.
The car was probable a northern car and driven during the winter in the salted ice/snow. Can also be a car that was driven near saltwater. Can also be a car with flood damage.
Fortunately, the damage is reversible.
the bottom of the car that you cannt see unless your under the car itself
That would be determined by the claims adjuster. Check with him/her.
It is advisable to wash your car at least twice a month. I pressure wash not only the body of my car but also the undercarriage, to avoid it from rusting. I use an undercarriage cleaner from best car gurus, makes it super easy to pressure wash my car's undercarriage without having to crouch down or lift my car
This answer depends on what damage the undercarriage has. If it is just minimal rust the it can be left with some anti rust protectant. If it is bent a body shop will need to attempt to straighten it out.
It is completely safe to use a pressure washer to clean your car as long as the machine you are using is suitable and not excessively powerful, and that you ensure you do not hold the jet of water too close to the paintwork, or drive water into sensitive areas such as window seals or corroded bodywork.