They typically make rust and corrosion WORSE, not better. They remove factory lee plugs to spray their anti-corrosion goo in, then often fail to reinstall them or do it poorly. Their goop can clog the engineered drainage paths and cause water to accumulate and sit. Some places even drill additional holes thru virgin metal to access hidden areas, which damages the finish and exposes unprotected areas.

That depends on the state of your paint. If the paint is still good it will help a bit because the coating will take some of the wear. But if the paint is already damaged then rust will have started and applying a coating on top of that won't make much difference, though it may slow things down a little bit.

The best protection against rust is to keep your car clean, especially from leaves, bird droppings and other debris. Wash but not too frequently and if there are scratches or other minor issues fix them immediately.

Yes if you use a liquid one like Fluid Film. No if you use a “hard” coating like Ziebart.