The amount of force that you have put into it should have been more than efficient to (if neccasary) shred the nylon. What is more likely is that it is the threads themselves that have been damaged from the 'grollying' as we call it. If its sitting flush, Id suggest that it hasnt been cross threaded, more likely that the the bolt had dirt/metal fragments when it was inserted and tightened, so if you cant get it undone with the force you are using, you need to get something bigger, or get the drill out.
Another point to consider is that if the bolt is 'upside-down' then leftey loosey is the other way.
Personally Id never get the blowtorch out for this, too much risk or damage elsewhere. Rather take my time with a drill and some decent tungsten-carbite bits. Welding a socket on may set you back even further, especially if you have already knackered a hex. Im also not sure melting the washer will work, I believe the threads to be holding the tension not so much the washer, and of course with thermal expansion you may have to wait for it to then cool.
Alot of basics here, so im sorry if it is too basic for you.
