Your comments are very interesting, and I'd like more input/thoughts about such issues.

However, from your description of the Alice/Bob part, I'm still confused whether you realize that nobody is (or can be) quarantined in the proposed scheme?

The case you describe would, as I understand it be: "Bob is then advised by his own app not to visit his parents and be careful about not infecting others". A bad joke, sure, but not something that can "send him into quarantine", since nobody on the server side knows that he is in any danger.

I may be missing your point here - but if I am, please clarify and bare with me;-)