Your system is really creative. Especially the 1-way syncthing with disappearing files on the host vm after the audiobook gets returned. That’s pretty cool tbh.
So I used Firefox and logged into mylibrary. overdrive .com and then checkout/play an audiobook in the browser. While it’s playing went to More Tools → Web developer tools →Network and filter by Media. The mp3s pop up there. Copy/paste the super long url of the mp3 media file into a new browser window and download. When you’ve finished downloading it, play it locally on your machine and your audio player will tell you how long the file lasts. Then click to the corresponding chapter to download the next mp3 file in the grouping. So for example, if the first mp3 lasts 1hr5 min, look at the audiobook and you’ll see that Chapter 5 ends at 1hr5min. So you click on Chapter 6 in your browser to begin downloading the next mp3 file. Repeat as needed until you’ve downloaded all the files in the book. I usually rename the files File 1, File 2 and so on so I know the order. The book I did yesterday had a total of 3 mp3 files.
Then you can put those mp3s into an M4B container with the proper chapter breaks (chapter breaks are conveniently timestamped in the audiobook web UI). You can screenshot and refer back to it later. If you want, put in some cover art into that container too :)