So I'm assuming that this is your test book?

I have not played with the Kindle 4 PC method in some time. It sounds like you set everything up correctly, so my only thought is maybe that the book's publication date is fairly recent, and perhaps Amazon has changed something with the way they ship the metadata file such that Calibre/DeDRM isn't seeing it.

It's in Calibre as an AZW3 filetype and I CAN open it in the Calibre E-book viewer - scrolling the mouse wheel moves me forward and back through the book but there's not a single icon or title bar or way to navigate.

So you confirmed that the book has been correctly decrypted, right? The text in it is readable? If it weren't, Calibre would throw a pop-up error thjat the book is encrypted and/or all of the text in the book would be garbled.

Unless anyone else has ideas, there's one other thing you could try. You mentioned you had a physical Paperwhite, so I'm assuming you should still have the "Download and Transfer via USB" option on your Amazon account when you go to My Content and Devices -> Books. Try downloading the book that way (hopefully it gives you an azw3 file--if it gives you a kfx, then we might be in trouble), and then in Calibre, go to Preferences -> Plugins -> [dedrm] -> Customize Plugin -> Configuration: eInk Kindle eBooks. In that screen, add the serial number of your PaperWhite.

You should now be able to decrypt the ebook that was encrypted to be delivered to your PaperWhite. Try importing that book (the one you downloaded earlier through Amazon via your browser), and see if it still has the same metadata problems.