Two options:
1. Place metal shield to protect wood after wetting wood.
2. Cut and cut and "pie cut" nut with saw and use hammer and chisel or air chisel to complete. After slicing nut a few places, maybe spray penetrant into slots and use socket again. Wish I had a few pics of the nut and area.

I assume by "heat" you don't mean cutting torch, just a torch. Just a torch is less likely to burn wood that has a shield. Bend and shape the shield to fit. Air gap between shield and wood is good.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn