If our Panasonic microwave shuts off after 2 seconds, is it likely a door latch or magnetron issue?
This is indeed a door latch issue; however, you're best off referring this to a microwave oven technician (they exist, although much of their business comes from foodservice not residential).  Microwave ovens contain a dangerous HV transformer and capacitor: the transformer will supply several kilovolts at several hundred mA, and the capacitor is well over a microfarad.  Furthermore, the door latch interlock is a critical safety system on a microwave oven -- if it fails or is bypassed, you can get into a situation where the microwave tries to start cooking you as well as whatever food you put inside!