As far as I can tell, if you've not already created the backup from the Datastore Admin to a GCS bucket, you will not be able to restore it to another application as there will not yet be anything to restore. Given that
copying entities to another application has been decommissioned, you may no longer be able to entirely copy the Datastore to another application using the Remote API.
As the Datastore Admin backup is essentially a use of MapReduce to preserve your data, you could essentially make your own backup process that manually copies over all entities. This may be involved depending on the complexity of your Datastore structures, but can be done immediately while your Datastore Admin production issue is being investigated.
Hope this helps!