Removing App from Play Store (Unpublish) - What happens with the users that have the App already installed?
No, the application is still available for users who installed the play and only store can not be reinstalled. By Play Store

If you put back the google play, users can download and install it again.

But never, under policies of google, an apk can / will be deleted from the device remotely by google / application owner

But we had a case in the past, about a malicious app that was removed remotely without trace, all done and confirmed by Google itself