Is it possible to downgrade Boot ROM SW to try and fix unbooting Late 2012 iMac from Single User Mode?
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.