glenrunciter - this is excellent info, thank you!
Did you also try the S812 MXIII release published in this post, with positive / negative results?
If only I could find any compatible firmware for the CX-S812 (S812-H chip) in .ZIP format, I would use the Amlogic Bootcard Maker to restore from bricked attempts.
Because I've searched dozens of hours and never found anything I can use in conjunction with the Bootcard Maker to create a restore flash, I have never been brave enough to try non-compatible releases as you have done here.
My biggest concern is the WiFi. I'm configuring this for the in-laws (I have also upgraded to s905 boxes myself) and the Kerber version's WiFi is not working good. Are you using a hard-wire LAN or WiFi with that firmware?
Thanks again!