I just checked FCOM (available Braking Schematic): "Depending on the brake pedals’ demand, the ABCU controls the alternate brake selector and the alternate servovalves."
Both valves are in line and supplied from yellow system pump and accumulators through the same line.
Only parking brake control valve bypass those 2 valves and can transfer either normal yellow pressure or accu pressure to the brakes.
It looks like that if you can't control those 2 valves, and you can't without ABCU (alternate braking fault), the only solution is parking brake. All that of course only in combination with Normal brake fault.
Following that, in case of ABCU and Normal braking failure you will not get much by putting nosewheel steering switch to OFF because everything, related to brakes, that it controls is already unusable.