Microwave oven blows internal fuse when door is closed
Years ago I had experience with helping an electronics shop owner next door to my office fix a similar problem. The problem turned out to be alignment of the micro switches which were all on plates with adjustment slider holes. On his unit the door interlock micro switches had an interesting contact arrangement with wiring such that when the switches sequenced incorrectly a direct short would be imposed across the power line through the fuse. This would cause the fuse to blow and render the unit non-operational. Best we could determine it was designed as a safety interlock system. It turns out that we guessed correctly.

It is a requirement that microwaves have an interlock monitoring system to pop the fuse if the switches are not operating correctly. There is a good description here:

http://www.microtechfactoryservice.com/switch.html