Try buying the actual OEM part. I would get from a local Appliance parts dealer. In Seattle I use Apex Appliance on Aurora.,
Inspect it at the store and see if it looks materially different? If the wire you say is melting is larger, that could be your issue.
I also wonder if the microwave itself could be cooking the switch? It’s an odd theory, but a switch doesn’t usually have high voltage, its usually just a gate for the rest of the circuit; If SOMEHOW the microwave was leaking radiation onto the switch, that could be an explanation too. wrapping the wire in foil tape might help this. Just crazy thoughts, I know.