Maybe I'm missing something now
There is no NWS gain control in the Cortex. There is 'rudder' gain that controls the gain of everything in the Yaw axis; this could be rudder(s) or NWS. You can have several rudder/NWS servos plugged in to the cortex but they are all controlled by the single 'rudder' gain. If you set this to zero then you turn off gain for all servos operating in the Yaw axis, including the rudder.
If you select the relevant channel for the NWS steering 'off', using the 'channels assigned' option in the software, then you can retain your rudder stabilisation whilst turning off the NSW stabilisation.
I hope that is clear but I'm concerned that I'm missing something now