Okay; after trying
dozens of Windows apps (I'm serious) that claim to be audiobook readers, I've finally found one that:
1. Has no trouble playing an m4b container containing mp3 chapters.
2. Displays a chapter list that's selectable.
3. Actually
remembers the last played position without having to manually set a bookmark.
It's called PotPlayer and may be found here:
https://potplayer.daum.net/
So I think I've found my system to manage my audiobooks without having to store thousands of separate chapters as mp3's.
1. Use NCH Switch Sound File Converter to create the m4b container and fill it with the mp3 files (chapters).
2. Use PotPlayer to play the m4b on my PC.
3. Use Sirin to play the m4b on my Android-enabled Kindle Fire. (Sirin supports all three reqiurements as listed above.)
I hope my journey might help other readers. Cheers!