ok lets look at this logically
The plate is static and doesn't move.
therefore the plate is a cover. It cannot move and come into contact with anything.
Therefore the plate did not cause this it is simply a "casualty"
Something underneath the plate malfunctioned and shorted across to the plate.
Microwaves are not really a repairable items.
especially as it's an old item, I would play it safe and simply get a new one.
Does it really need all the bells and whistles,?
Did you use all the functions on all the settings or just defrost and full heat?
you can get them a lot cheaper if you don't go for the ones that can do everything.