Mark,

One thing that stood out at me is that the notices sent to Moderators don't carry a hashtag.

This means that group activities (e.g. File, Photo, Wiki changes) create two different types of notices: a Group Notice and a Moderator Notice. The Group Notice is optional (as you've noted), has the corresponding hashtag, posts to the group's messages, and the web/app is opt-in. The Moderator Notice is not controlled by the person who took the action, does not carry a hashtag, is not stored on site, and is Opt-out for both email and web/app.

Member Notices (generated manually via the Members list, or automatically via the Member Notices tab of the group Settings page) are like Moderator Notices in that they do not carry a hashtag, and are not stored on site, but they cannot be opted-out by the member.

You also need a row for "Message posted in a Topic with a hashtag" because they act like Group Notices too: the member can opt-in for a web/app notification of such message postings. Yup, any old hashtag, not just the system-defined ones.

I haven't done a cross-check with the Notifications page in GMF's Wiki, but that may turn up something if you haven't done that already.

Shal