This is workable except for the mixer. The docs say: "Module A-138p is the 4-fold input module, A-138o is the output module that can be combined with one or more A-138p." You are lacking the necessary output module.
You could replace A-138p with an FBP 6 ch mixer. 6 inputs, stereo out, head phone amp, and an fx send to, too. Plus a cue function for pre-listening.