[xyzzy_foo]

Alright, so I did some digging into BookFab Kindle Converter. I completely uninstalled and cleaned up both K4PC and BookFab, then observed its behavior both online and offline.

First off, BookFab requires an internet connection to start. This is because it needs to communicate with its license authentication server. If you launch it offline, the app won't be authenticated and you'll be logged in as a guest.

Next, while BookFab no longer asks for your Amazon credentials in its built-in browser, it does require them to be fetched from K4PC. This isn't for a .kinf2024 file; BookFab doesn't need that key.

Then there's the conversion process, which absolutely requires an internet connection and involves communication with Amazon's CDN servers. A VOUCHER file is used during this process.

Finally, outputting the file is done entirely locally. It uses the KFX files that are already on your PC, so you'll get an error if the RES and MD files are missing. And of course, the AZW file is also required.

So, in conclusion, the current version of BookFab does convert local files, but it communicates with Amazon's servers in some way to handle the DRM. This means there's a definite possibility of your Amazon account getting banned. However, since it doesn't use a key, you might be able to remove the DRM from your own files whenever you want. It might even be possible to strip the DRM from someone else's KFX files, but I have no desire to test that, and there's no telling what would happen.

BookFab is kind enough to generate detailed log files, so someone with more technical expertise than me could probably investigate its behavior in greater detail.

TL;DR: I strongly recommend sticking with either the Satsuoni KFX2024 method or the my K4A KFX2018 method. Both of these handle DRM entirely offline.
By the way, regardless of which method you choose, the quality of the resulting KFX file will be the same.