if a calendar meeting invitation was sent out to other people, do those people then see the callback URL after an Agenda note is linked to that calendar event (in the notes/description field)?
No because if an event has invitees we don’t add the Agenda URL to prevent exactly this kind of confusion. However, if the invitees were added in the Calendar app after the event had already been linked to an Agenda note, then yes, the invitees can see the link.
Or is that URL just for the local user?
No, in the latter case above the url is visible to anyone having the event. However, please note that the URL is added to the notes field, not the URL field.
The corollary…if the calendar event is one that I received from someone else (i.e. I’m a recipient), and I link & add my notes in Agenda ahead of the meeting…if the meeting organizer changes the meeting & sends out an update, does it over-write the callback URL?
No, because of the fact that a) it’s in the notes field, not the URL, b) we don’t add the agenda URL to such events because it already has invitees (see first question above), and c) in case some combination of circumstances does lead to two Agenda copies adding a link to the same event, we do things in a way where both links are preserved in the notes field.
Hope that answers the questions!