btrfs check said broken refs and some other errors,

Gotcha. That must have been a kernel bug (or hardware error), none of the userspace utilities could cause it unless they were trying to manipulate the block device directly, which would be really dumb. It’s possible it wasn’t even related to the subvolume manipulation.

I don’t see how systemd has anything like the Windows registry. At least its journals are leagues ahead of Windows event logs, I hate those things and the awful viewer they have.

btrfs data rescue after I deleted a parent snapshot of my rollback

Can you expand a bit on that? I thought it didn’t matter if you deleted parent snapshots because the extents required by the child would still be there.

Gnome isn’t mentioned anywhere in that exchange?

It’s a very well known historical story though.