The real problem area are sharp points that concentrate the microwave
energy and act as lightening rods and various small pieces of metal like
twist ties and metallic trim on plates which will overheat.

This is exactly it. As long as there are not regions of significantly
different potential, there will be no arcs. Smooth transtions will not
cause arcing.

If you want to prove it tear off a piece of aluminum foil and stick it
in the microwave. Nothing will happen. Now take the same piece and
crumple it up and flatten it back out loosely. There will be all sorts
of wild arcing across the peaks.

JW