In the UK, the best scratch repair stuff is T-Cutt. It's available in many colours (including about 8 shades of white, ranging from 'Ford White' to 'Ice White/Mondeo'...).

We've never noticed cats sitting on our car, but then we park on the street. The piano on the other hand....

Mostly, a cat will only scratch a surface getting off it if they are scared off or startled. Turning a hose or a water pistol on them will do this and they'll scratch your car.

As for keeping cats indoors - nature didn't design them for sitting on the sofa all day.

LTS