After publication of the article above, I encountered another issue that I’d seen before, but had assumed was something I was doing wrong. Nope: Photos cannot show previews of adjustments and enhancements in real-time anymore. Sometimes it works; sometimes it doesn’t. The solution, according to Adjustments not working in Edit mode using Photos is that “[q]uitting and relaunching the application fixes the problem temporarily.” What a lovely workaround. This issue has apparently been in Photos since it launched in April. No fix from Apple in sight.

At least a restart did solve the problem, as advertised. I hope it survives without further restarts for the 60 pictures I want to touch up.