Thanks so much Red (and all).
So the .BAK will work. I will start implementing that for the base original file.
But the Evernote use - which I have become ever more using - is an interesting idea. I will give that a try. Same with OneNote. That I think will be a great way to approximate what I am describing.
I
also never thought about a LINK in the DESCRIPTION to a FOLDER for that author/book - for supplemental materials. I think I will experiment with that as well. I could also do - as you are - by placing a link to this SUPPLEMENTAL folder in EN. But think that this one in the description would also be a possible good approach - as its direct from Calibre (one step) rather than via EN as a "middle" third. Perhaps you might find that something of interest.
Thanks again. This was VERY helpful (and I am now filled with optimism for at least ONE thing today

).