Since when do microwaves operate with the door wide open?

On some MW's it's as simple as tapping the door sensor closed. Same with dryers. Not all of course. But enough to make the scene reasonable.

Or maybe... yes, advanced circuitry training in addition to years of medical school?... totally believable. :)

The microwave was broken, i.e., did not work correctly. That flittering vital detail was noted when they first arrived at the house.

So yes, you are The One.

I was less concerned with how plausible the scene was than how silly it was. The rest of the violence in the film was brutal and harrowing, and then out of nowhere we suddenly get a ludicrous "Itchy and Scatchy" moment with an exploding head in a microwave. The hand in the garbage disposal was a bit silly as well, but I was willing to let that one slide. But the microwave scene took me right out of the film.

Just go with it. It's a movie.