(2) When you heat the bolt, you send the heat down the bolt. This
causes it to expand, and then it shrinks as it cools, loosening the
binding. Also, expansion and contraction can be very local.

That's what I do on outboard motors with alum blocks etc. Very difficult to
drill a stainless bolt stub out of an alum block (. I then let the bolt cool
and before attempting to remove it, heat the area around it to expand it. Then
I try the bolt. Usually works on the first or second try.
Greg Sefton