I figured out how to finally get this to work. For Mac OS, you have to install an old version of the Kindle App. Version 1.23.1 appears to be the last version that downloads AZW3 formatted ebooks that can then be converted to EPUB easily with the DeDRM plugin. All later versions appear to be broken. Also, the Kindle app will silently update to the latest version unless you turn off the auto-update option. I converted all of my paid books to epub and not buying any more through Amazon. This post inspired me to do this before it is too late and the old version is no longer compatible to future OS versions.