Yeah, I have the same controllers too. I think the “Sel” switch will need some sort of custom mapping or at least some dedicated MS recognized function for the “knob press” that is found on Boeing MCPs. The hard part is that the IAS/MACH - HDG/TRK - ALTITUDE selector would change how the “SEL” switch functions, so not sure how to even set that up to beginwith. Which function are you trying to use it for? I personally would use it for HDG HOLD / HDG SEL only, which I’m not sure is an option in FS.
Speaking of, does your rotary (button 15/16) work correctly? Mine is really slow or jumpy, which is a bit indicative in the control panel, since the button presses displays a lot faster than a normal button press.