Is it possible to downgrade Boot ROM SW to try and fix unbooting Late 2012 iMac from Single User Mode? I currently have 283.0.0.0.0 Boot ROM SW installed on my crippled Late 2012 iMac - which I upgraded HDD->SDD, tried to unsuccessfully install Mojave, then broke fusion setup, erased drives and am now stuck (see other questions on that...)

Specifically one thing I would like to try is to use the last standalone EFI update dmg from Apple support website Apple EFI and SMC Updates for my model  (iMac13,2)

I can only access Single User Mode - I don't make it as far as Recovery with Utilities and Terminal options.

Is it possible to do this from Single User Mode? I can put files onto a Mojave or High Sierra external image which I could then access but I don't know enough about what commands and scripts to use to update firmware.

Thanks
The panic report you posted in your previous question indicates an unrecoverable error in the IOAcceleratorFamily kext while trying to communicate with the NVDAResman kext.  This indicates a problem with the GPU, probably related to the weird firmware update you applied.  I think you're out of luck at this point.  Only Nvidia has software to downgrade a GPU ROM version, as far as I know.