I appreciate your help so much. Can't say it enough.
I moved the files you mentioned and no longer saw a corrupted piece of backup happening. Yay! But then when I connected to Calibre I got this error (see pic). The main point being that it still doesn't backup annotations, reading progress etc. So I'm unclear what I corrected. (that error only popped up the 1st time I connected though)
To avoid issues in the future... I'm very careful to disconnect from the PC but you mentioned waiting for the "connected and charging" screen to disappear from the Kobo. This doesn't happen on mine. PC says it's safe to disconnect and I can wait for 5 minutes even and the Kobo "connected..." screen never leaves.
(Kindle was instant so surely I'm waiting long enough) Naturally I thought that screen simply stays in Kobos.
(Paperclip is very handy. Shame it's hidden. I like your settings for once a day5 copies & switched. I'd followed the advice of someone online when I first set it up. They did 10 a day so I went down the middle. LOL)