hammer and chisel the bastard off without fuking up the thread but by the look of your previous butcher work maybe let someone else without parkinsons have a go at it.
My offerings on the scenario are either the chisel method or get a piece of solid rod (scrap metal yard) and grind it down to perfectly fit the "shape" you've created. The closer a match you can get to the actual hole, the less likely it will slip. Then cut it off so it's only about 2 cms protruding out of the nut and grind a hex onto it so you can grab it with a single hex socket.
When you're talking heat, you don't want to heat the nut, you want to warm the housing around the nut to make it expand away from the nut. it should work ok too as the alloy will expand at a lot faster rate than the nut. Don't leave it to heatsink for too long though. Heat it for 30 secs then try straight away.