Are the IDC connectors making good contact on CN14 on CPU board?

You've checked continuity, so I guess we can rule out a wiring issue from at least the diagnostic button to CN14.

Maybe if you can swap the CPU board from the spare GNR you have (lucky bugger) and see if the problem stays with you..

Now if I'm reading the schematics correctly - for the right switch which is not working - that should be the up/down button which is pin 4 on CN14.

Check R8 and R7 for starters..(Ohm) and what voltages are you getting at R8 & R5...Should be close to 5vDC...

I just checked my SW and it was about 4.88 vDC