Before throwing my hands completely in the air I might try swapping Z and X optical switches but not changing pins where they go on C23 BOB, and changing pins in Mach. If the problem moves to X I have one thing, if it stays on Z, I have another. However, I'm not sure I know what to actually DO in either case!! My uneducated guess would be if it moves to X, it has to be the CNC4PC C16 home/limit board, if it stays on Z it has to be C23 BOB or SS..........or Mach somehow.
So....finally got 'round to swapping physical opto switches, Z for X, X for Z. Switched Ports & Pins to match. The problem went to X. It won't back off the switch on X now and it DOES back off Z just fine. In my mind, this makes the CNC4PC C16 the culprit. I consulted Arturo again and he's looking into it a bit before sending me another board/switches to try.