Your topic is interesting :D lol, but thanks, i came to help only..
Did you recalibrate your TX in the mission planner, before u do this make sure the channel 5 is functioning, for exp: simply connect a servo to ch5 and see if it moves, if so then do the calibration,, Because i had a similar problem that the switch is not working in the plane, but the mission planner shows the modes are changing, so i re-calibrate the radio in the mission planner so the APM will read ch5.
you need to remove the pot: having the mode switch and pot in either series or parallel is asking for problems. To test with a multimeter, the easiest way in this case is to assume that the Vcc wire is Vcc, and check to see which of the other two has the lowest resistance. Whichever it is will be GND, the other SIG.