I have to temporarily unpublish my app from Play Store for legal reason until I sort them out. What happens with the users that have the app already installed? I understand they are not allowed to find it from the store or install it, but does it also mean the apk they have already installed will be uninstalled from their device?
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