I just wanted to post my personal experiences with Kindle-for-Mac and Catalina, and to thank user odamizu for this post --
https://www.mobileread.com/forums/sh...&postcount=508 -- which solved my current problems.
I had been using Kindle for Mac with Calibre successfully in previous OSes, thanks to the chmod technique posted in this forum. But when I updated to Catalina, that ability broke.
But re-entering the chmod command (no need for sudo) and following the right-click downloading technique recommended in odamizu's post did the trick. Now I can successfully convert in Calibre once again. Kindle for Mac version 1.26.1
Thanks, odamizu! Thanks, forum!