While I continue to try and determine what's up with my CHESSmate firmware, I'm forging ahead with the hardware. 

Hardware

I'm reasonably certain that the CHESSmate PCB and ESP-32 are working as expected from an electronics perspective. So to that end | have designed a daughter board to attach the ESP-32 to the back of the main board via the 22-pin header.

The idea is that the daughter board will have a 90 degree female header underneath and facing "inwards" to connect with the existing male header on the PCB like a little "back pack". My original thought was to redo the PCB and add the ESP-32 to the board itself, like Oscar Vermeulen did with his KIM Uno project. In the end I decided this way was more flexible, especially if I (or someone else) ends up doing a 6502 based hardware "back pack".

Software

On the software side I have tried a few time to play a game through on my reproduction. At first everything seems to be going great. But after five or six moves CHESSmate signals that a move I have made is invalid even when I know it's a good move. There is an option to check the positions of all the pieces on the board (B - ENTER) and when I do this I find that CHESSmate has lost track of one or two of the pieces. Sigh. I have the hardware now to dump the ROM from my original CHESSmate. I sure hope that is the problem because if the ROM is not different from the ones I download I'm not sure where to go next.