"frank1492" wrote in message
Having no success, I ground the head
off, thinking I could then liberally apply penetrating oil and
eventually get the bolt out by turning it with a vice-grip.

At least you learned a lesson



Would it be worth trying to heat the bolt with a propane
torch? How should this be done, and what would the risks
be?

Just heat the heck out of it and then try the vice grips again. Get it as
hot as possible and aim the flame right at the bold.


Would there be increased risk of shearing the bolt off
after heating (assuming it still won't yield at the threads)?

No. Heating is pretty reliable as long as you don't burn anything around
it.