No, you cannot use it as a tabletop model as built-In microwave should be built into a cabinet as the sides are exposed without any paneling. Therefore, the exposed sides of the microwave will be very hot when used. The same applies to tabletop model as it cannot be used as a built-In model.
No need to preheat the microwave if you are just using it for reheating purposes. You only need to preheat the microwave if you are doing convection cooking.
When the appropriate temperature has been reached, the beep will sound twice and the temperature set will flash on the electronic LED screen to remind you to place the food into the microwave.