"Butchery autopsy report on the Tx? "

Good idea. Also you end up ith some magnet wre for the juk drawer. Comes in handy sometimes.

"Bifilar wound hotspot at the most insulation-stressed point? "

Plausible.

"blown
thermal fuse? failed PbF coilwire/tail joint? "

Not if it was pulling excessive current and frying the thermistor.