Hey RG, interesting video. So Y cable (TRS to 2 x TS) is the go, TRS at the rockmaster and Tip to switch 1 and Ring to switch 2 (or vis versa). This emulates the 2 switches on the peavey pedal and turning them on/off on the FCB will be the same as pressing them on/off on the peavey pedal. The peavey pedal has 4 states:
Tip off Ring off = FCB 1 off, 2 off = Bypass?? (I'm just guessing what the rockmaster state equates to here based on your peavey FS diagram)
Tip on Ring off = FCB 1 on, 2 off = Clean??
Tip off Ring on = FCB 1 off, 2 on = Ultra??
Tip on Ring on = FCB 1 on, 2 on = Ultra/Crunch??
So since you can assign multiple things to each patch, you can include a different switch state in that patch e.g. Tip on (FCB 1 on, 2 off) etc. So when you change patch it will select the rockmaster mode/state as part of the patch. I'm assuming this is how you want to use it? Alternatively you can program the FCB up/down buttons to toggle the FCB switches. I can't see that you would want to use momentary switching (which the FCB can be programmed to do) unless you want to press a FCB button to change the rockmaster to a different state and hold it, releasing reverts to off again. So latching (which is default) makes more sense.
Using a TRS at the FCB end doesn't do anything practical, the FCB switches just toggle the tip hi/low, on/off and if you had a TRS plugged into it, it "probably" toggles both the tip and the ring
at the same time (guessing here) which would/may cause strange results at the rockmaster end.
When you include the FCB switching in a patch, you need to include both FCB switches and set them to either on or off to ensure you select the right state on the rockmaster. This may be why you have been getting strange results?