if app is unpublished then is will not be available for download, will not appear in in-store search results, and simply cannot be installed from store. No details information from Google Play can be received even with direct URL to the application. Unpublishing however does not affect existing installations (so apps will not be automatically removed from any device that got it installed already). Also existing users still would be able to update if they are on older version.
And you cannot have another application with the same package name, no matter what.
EDIT
Please note that you unpublish (and this means just what name says, nothing more) not just APK - you hide all related stuff (app's page in Google Play etc). To end user it looks like the app is gone. For you - it is still in your console, you can edit description, images, see comments, stats or update APKs etc., but unless you publish it again, nobody will see these changes.
