Me again:
I identified a version of DeDRM Tools that worked with my older Calibre: version 6.8.1. I added that to Calibre without difficulty.
Then I realized that method 4 did require a functioning older version of Kindle as well... Back to that, then. I installed Kindle 1.24.3. But when I tried registering it, Internet Explorer went nuts: two windows kept opening and closing, and the logon window kept reloading, so I could never input my information.
Giving up on method 4, I moved to method 2 b): I reinstalled Kindle 1.33.62002. I was able to re-register it without any difficulty. Then I launch the BAT file found
here. Then I re-downloaded my book, imported it into Calibre, and tried to convert it again.
Well, it failed again, but this time the message is different: "Conversion impossible - this book is locked by DRMs."
Any idea where I can go from there, if anywhere?