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