I made no additional changes, but somehow it’s working correctly now. Perhaps the system had to sleep on it or something, but when I checked again this morning everything was working properly.
I believe that these instructions do work:
Why are the comments disabled by default on my custom_post_types?
Essentially kick-starting the comments in SETTINGS > DISCUSSION, much like clicking “save changes” in SETTINGS > PERMALINKS resets the system if you run into a 404 after registering custom post types.