so your unable to unscrew the 12g. knob while it is aired up because it will not fire, to release the pressure,correct?
I'd just use a piece of leather or something to protect the 12g. knobs knurling then use pliers to break it loose..the 12g should degas after a a 1/2 turn or so.
there's a vent hole in front of the valve for the co2 to escape through.
don't unscrew it all the way