Hello Linda, sorry to be so slow in responding, I was too confused to be able to add anything to your investigation.

I now find that manually "subscribing" generates e-mails when there are posts to the thread: no matter what my user profile says for "Use 'Email Notification' by default?" and even if I don't post in that thread.

I am longing for the good old days when posting auto-subscribed and only PM's generated e-mails (because that was what I asked for).