For anyone that may come across this later…
I found that I had missed setting the module name within my custom .mu file itself:
At the top of the file there is a line like this:
module: shotgrid_fields_config_standard
which when renamed to:
module: shotgrid_fields_config_custom
works without the syntax errors I found above.
I didn’t check this with behaviour of using a named custom fields file, but I imagine it might account for the same issues that I found there as well.
Additionally to reset the Config Style option it does seem that <PATH_TO_RV_EXE> -restPrefs
does work to do this, but that does removed all preferences so I’m still not sure how to target resetting just the Config Style option.
But hopefully this helps anyone else in the future