And finally it is behaving the way I expected. Evidently I needed to enable mailing list mode in my user profile in addition to enabling it as a site administrator.
I genuinely don’t know if I read that and failed to notice it or it’s not spelled out enough in the various bits of documentation.
I definitely needed someone to say “Do this” and for it to work. I appreciate you for doing exactly that.
Even so, it would be very convenient to be able to generate a reply key to an existing topic that I wasn’t Watching when it was created, so that I could start replying to it by sending email. This seems like a non-obvious-but-helpful scenario to me.