I don’t have a direct answer to your question. But I would like to offer my opinion here for question #3 “just purchase Obsidian Sync”:
Obsidian Sync (or any sync) should not be a replacement for full (or multiple redundant kinds of) backup. There are still some instances where people run into sync problems with Obsidian Sync, iCloud, or any other types of syncing services.
Even git and Github can have issues, as evidenced by the colourful language of this website: https://ohshitgit.com/
(Just be careful about relying on any one thing.)