Congrats on your 12X purchase; I love mine and I wish you many successful flights with yours.

Regarding the rudder & nose wheel steering setup, this is the setup that I understand is typical.

Hook up rudder(s) to rudder channel
Hook up NWS to an aux channel (lets say Aux 4 for now)
Using Device Select, assign the L Trim to Aux 4
Mix rudder to Aux 4 using a mix activated by the retract switch

As for what to do with the center knobs, here's my selection