This is the other problem. You will need to use two separate browsers, like Safari and Firefox, or Edge and Firefox, because a single browser will usually share the identifiers across tabs.
As for the first problem, you could make your admin function write a file to disc, let’s say a json string which contains the reason and the time when the update will be applied. Then add a Timer to the App class which looks for the file once a minute. If it finds it, it notifies all of the users that the app is going down, why and when. In app.opening you delete the file if it exists.