Good News FINALLY - after performing the NVRAM reset and proceeding to reinstall Catalina from USB onto a USB partition AND taking all of the Apple updates, I can report my EFI firmware was successfully updated,
View attachment 1959374
(*above screen capture was while booted into Catalina)
Again, I did not touch the internal SSD with OCLP+Monterey and the EFI firmware persisted when booted into OCLP+Monterey. Woohoo!!!
SL booted from OCLP+Monterey still shows EFI firmware at 425.0.0.0.0 with "SIP enabled; SSV disabled".
Wow, that's really great to hear. I'll try that now on my rMBP 10,1