@paul, delighted to hear my tedious novella was useful to at least two other people. Makes me feel less embarrassed for having written it.
@andrjohns, thank you so much for this fix! This came up at the end of the other thread too, but it wasn’t clear to me how to permanently edit the package Makevars and I didn’t follow up. I tried it, and it works! Without the extra user Makevars flags it does still puke out a lot of warnings to the console, and (as a result?) takes ~ 15% longer in my case. But the important thing is it compiles successfully, and this way I don’t have to instruct users to add all those -Wno flags, which is obviously much better.