Loss of paint in the cavity (the cooking area) of a microwave oven isn't a safety problem unless that spot is rusting and has rusted through. That will leave a hole in the cavity, and that is a safety problem.

The working part of a microwave is a cavity magnetron. In essence it converts electricity into radiation of the microwave wavelength. The radiation then excites the atoms in the food and it becomes hot. The Cavity magnetron was also a vital invention in the history of radar, without it the Allies might have lost the second World War.

the microwave tubes that undergoes velocity modulation are klystron tubes ie, two cavity klystron.....

Yes, you can heat air in a microwave oven, but the appliance isn't good at doing that. There are a few reasons for this. First, the microwaves will most effectively only heat the water vapor in air or water in something in the cavity (the cooking area). That means that thermal energy transferred into water vapor or something in the cavity that has water in it will have to be transferred to the air to heat that air.

We also know that a microwave has a fan in it to keep the microwave generating component cool. The air is heated as it cooles the device, and that warm air is then ducted through the cavity and out another side of the oven. This happens continuously. The air in the cavity is constantly being "replaced" with other air. Heating air in a microwave is not something the device can do well.

Microwaves are generated at the atomic level, based on the frequency of the EM radiation that it emits. This is conducted through radiation that is absorbed, reflected, attenuated, or refracted.