I have enquired with Shanling. The player can read M4b files as long as they are renamed to M4a. The player can save the position of the last file played. It cannot do this for more than one file.
This may work reasonably well for listening to books only. If you listen to music in between, you're lost... A pity, otherwise this would be an inexpensive, incredibly small second player.
For more advanced audiobook functionality, the Android players e.g. M3X or M3 Ultra were recommended.

Unfortunately I can't answer that. I have never tried it.