- By default it is Ctrl+Shift+8, as mentioned in the description. You can customize this through Firefox' add-ons menu -> gear icon -> manage extension shortcuts.
- Yes that is true. While unlocked Private Bookmarks behave just like any other bookmark.
- Feel free to ask any further questions on the issue tracker (https://github.com/rharel/webext-private-bookmarks/issues)

Firefox periodically backups your bookmarks. See if you can restore them through Ctrl+Shift+B => Import and Backup => Restore

Apparently it's a rare bug that pops up under specific circumstances for some users. I have not yet been able to reproduce it on my end so unfortunately all I can advise is to export backups of your private bookmarks once in a while so you can restore them if you encounter this again. Thanks for the feedback.

Thank you!