I haven't investigated updating of data using anything clever. I know the database load script can write out a list of tiles that need to be rerendered, which can be fed into the renderd process. Renderd has multiple priority channel so bulk renders are a lower priority than user views, so a live system can update the tilecache without much easier impact.
I haven't investigated it because we are probably moving to Vector tiles over the next month or two, and we don't need up to the minute map updates. On one of our sites, we have been rendering navteq tiles and we only get updates for those quarterly. It was only our public site that we were using Google Maps for.
Backup and restore we are just doing via normal backup programs. The tilecache is stored in meta tiles, which is a d duplicated set of 64 tiles in a single file. So we are only dealing with about a million files in a directory structure totaling 25 gigabytes.
If we needed to restore or update our tiles, we would probably just spin up a new box and rerun ansible to deploy it. I have it all encoded Within a Playbook.