Parts I'm less sure of / things that could use help
NB: This section may be outdated, I can't remember what the current status is since I wrote this originally a long time ago.
Personally, I feel pretty good about shipping this largely as-is (modulo any bugs or code review issues) and I don't consider the below issues to be blockers – if anything, they might be best addressed in follow-on PR's. But, I raise them here since I anticipate they may garner discussion and I'm not so sure that my current answer is one that I'll like later on.
1. Whether and when to shorten to "case-style" for non-nested ternaries
As written, the PR uses "case-style" when the consequent is visually short (under ten characters long), eg: