Unfortunately, you have a clearly corrupt NVRAM volume, no doubt. Some things to check and try:
- Check the RTC battery voltage, replace it with a real BR2032 if below 2,95V. There were cases in the past that when the RTC is not working correctly, you can't reset the NVRAM.
- Did you tried a deep NVRAM reset?
Sometimes the deep NVRAM reset can solve the 2nd VSS store NVRAM corruption, so, always try it first. If the 2nd VSS store is still corrupt after doing it, you gonna need the BootROM Reconstruction service.
To deep NVRAM reset, install a wired KB that works for NVRAM resets (not all KB models work), press and keep pressed CMD-ALT-P-R until you hear at least the 4th chime.
- Maybe you could also need a SPI flash memory replacement, one of the clues of a defective SPI is the no persistence of NVRAM variables. Obviously, the macOS install will fail if the NVRAM values are not being staged correctly.
You can backup the BootROM again, save it to your cloud storage, and then flash the MP51.fd temporarily to diagnose if the SPI is damaged.
Your Mac Pro won't have any serialization, won't work with iCloud/FaceTime/Messages or any app that require serials, so, this is not a solution but just a diagnostics step, once MP51.fd is flashed successfully will have a working NVRAM volume, so, if the same issue with the NVRAM entries not surviving still happens, you gonna need to replace the SPI.
Mojave 10.14.6 full installer have the MP51.fd:
Code:/Applications/Install macOS Mojave.app/Contents/Resources/Firmware/MP51.fd
If the SPI is really kaput, maybe you can't even successfully flash the MP51.fd, so be aware that there is some risk involved here.