Tell steam to verify the files. That should make it realize they’re missing.

Unfortunately, directory monitoring doesn’t work if your music is on an NFS share.

You go into the LXC’s console and type update, or use whatever package manager is available in the LXC.

An extra hard drive doesn’t save you from something like your house burning down. Off-site backups are important!

I’d look into tools built for that purpose. For example, vscode’s live share feature. Other IDEs have similar features.