All done (apart from parsing 36a) after a monumental 11 hours. My brain is frazzled. Nice to see that the two highlighted regions are also thematically placed. I don't know if Mountain Ledges is a new setter, but I look forward to the next offering from him/her.
You can work out 36. You should know what the letter generated by the clue is, and which word the changed letter is in. It might help to know that in “all of his”, which refers to the second word of the clue, is a one-letter abbreviation
I realise 'all of his' has to produce the s to knock out of fist. Can't look at the moment, but have spent some time trying to figure that out and I'm struggling.