Hi, thanks for the response.

No, I don't have "Run Cookie in the Background" checked.  I did read the Forum wrt the problem before posting, and saw that you make that point.

Re Safari not sending any notifications on tab closure, I had a very quick look on the weeb and fell over this page:
https://developer.apple.com/reference/safariextensions

It contains some events which might be helpful, eg:

Safari Activate Event: Instances of the SafariActivateEvent class are used to notify listeners when a tab or window is activated.

+ SafariCloseEvent:  Instances of the SafariCloseEvent class are used to notify listeners when a tab or window is closed.

Clearly, going that route would metamorphose Cookie into an extension rather than a helper app... but would that be a Bad Thing

Wish I could help, but am a Linux coder, Apple and its pretty-much-closed source aren't somewhere I wish to go, my life's too short 

C'mon, let me encourage you to go for it!