@turner It looks like you have worked out how to add the MAC address using a hex editor as I suggested, which is good.
If FPT won’t work you can use Gigabyte’s EFIflash instead:
[Tool] EFIFlash v0.80/v0.85/v0.87 for Gigabyte Mainboards
I would suggest using the modified version of EFIflash as you have likely changed the checksum of the official BIOS when you added your MAC address (which will cause the official EFIflash to reject the modified BIOS).
If you can downgrade the main BIOS chip to F5 with EFIflash, then you can also flash the backup BIOS chip using the command line switch /DB
E.g. efiflash Z97XSOC.F5 /db
This will flash the BIOS file to the main BIOS chip first and then the backup BIOS chip.