Let me start with your 3rd question:
What I called a local mirror is the fact that for normal files you see in your local OneDrive folder what is mirrored to the OneDrive server(s). Where mirrored = backuped.
Normal files means: all files, except OneNote files. They need a special treatment.
Your idea to copy the Documents tree to the local OneDrive folder to get a backup on OneDrive was generally clever; not knowing the special treatment necessary for OneNote files.
OneNote, from the beginning, allowed live shared access to its notebooks by more than one user. This started with using standard Windows features as Shared Folders in a LAN or WiFi network at home.
Since OneNote 2010 they introduced sharing notebooks over the Internet. For that they changed the data structures of OneNote in an incompatible way and invented a new solution for shared access to these notebooks on SkyDrive, later renamed to OneDrive. The
workaround for the incompatible data structures was to use the new file format only on OneDrive, while still using the old format for locally stored notebooks.
This is no problem when creating a new notebook on OneDrive, which is possible since OneNote 2010. But this is a problem if you want to move a locally created notebook to OneDrive. This step obviously makes necessary a format conversion. This missing format
conversion created your problems.
There is only one way to make this conversion: use OneNote 201x and its File -> Share feature to move and convert an existing notebook to the right format on OneDrive. A side effect of this conversion is that such a notebook now appears in the local OneDrive
folder (and in OneNote itself) as a hyperlink. Using such a hyperlink in your browser opens the notebook in your browser, in OneNote Online, to be exact.
As long as you use OneNote 201x, not the builtin OneNote for Windows (the OneNote without a File menu) you have a local backup. Where it is you can see in my screenshot.
More questions 😀 ?
Bernd