If a microwave oven with painted inside walls has some of the paint removed due to a very small fire caused by arcing, is it still safe to use?
Yes. The paint is simply decoration on the metal walls. The cooking chamber of the microwave has metal walls so that the microwaves will reflect around inside the chamber. Thick metal surfaces are mirrors for microwaves and they work perfectly well with or without thin, non-conducting coatings of paint.