@arjaybe -- Thank you so much for providing your audiobooks. Your narration is VERY nice, bravo!
On using Calibre to convert epub to old Mobi --- I have done so easily, and use the Kindle Previewer
https://www.amazon.com/gp/feature.html?docId=1003018611
to review appearance in several emulated devices. Not 100% foolproof, but gives a pretty good overview. If necessary you can go back and tweak the source epub (yay Sigil) and convert again until you get pleasing results.