Hi!

My theory is that the microprocessor was failing,
and that my cooling adventures made it fail
completely.

I've got my doubts. Thermal shock is a real possibility and could have
happened here.

I seams possible to buy a whole new control board
as a spare part, but it costs ~200 dollar, which
is more than I am willing to spend on a
microwave almost 10 years.

I'm sure it is. However, for that same $200, you could buy a very nice
microwave oven. And that's the unfortunate thing.

You might consider saving some of the parts from your current oven, as
some of them are useful, could make good spares or are suitably
dangerous (!!!) depending upon how you look at things.

William