So what happens if you forget to remove policy-rc.d and the server reboots? Does the PostgreSQL fail to come back up?
Is there any issue (does it happen and does it matter) if the upgrade replaces files underneath a running server.
Why doesn't the upgrade routine handle this itself already?
Second and third questions are more tricky:
so currently upgrade routine follows documentation (keeping data safe). Maybe in future, there will be "better" upgrade procedure, doing something similar to nginx (http://nginx.org/en/docs/control.html#upgrade), which probably will be issue with memory / resident set / caches.