Sorry I never realised that anyone had responded against this thread.
You've actually found a bug with the eog problems you were facing. The script should have been falling back to loading the captcha in the web browser if eog wasn't present. That's fixed now so thanks for highlighting it.
I've additionally added a section on how to actually transfer audible books to old kindles as its surprisingly hard to find any information on it on the internet nowadays:
https://github.com/kennedn/kindlepas...ing-Audiobooks
I hope that helps.