Lot of fuss about nothing significant on this thread, simply heat the locknut up, give it a good wire brushing to clean the threads, back the nut off, and undo the TRE, if it wont undo, heat it up till it does, the steering arm itself, shouldn't need heating, so any heat transfer will be insignificant, replace TRE and nut with new...
EDIT, A propane blowtorch is unlikely to work, as temps required will not be achieved, Oxy/Acetylene will do the job...
Thom