For this issue here is the answer from Google Play Team:
Hi x,
Thank you for your note. If you've lost your keystore you'll have to publish the app with a new package name and a new key. You should also update the description of the original app and unpublish it. Please note that users are allowed unlimited reinstalls of each application distributed via Google Play, an application will remain available to users that have already installed it even after it has been unpublished. Currently we do not support the deletion of apps or the re-use of package names. Unfortunately, there is nothing else we can do to assist you further with this matter.
Regards,
The Google Play Team