Steve, I know that the switch itself could not cause the starter to continue running, but there is no mistaking the sign that when I moved the shifter, the resistance readings changed. The Fluke 23 digital meter I use for car work is not the best meter I have access to, but it has always been reliable.

This car has had the manual transmission swapped out with an AW70 in its place - I am headed out to do some more tests, but I am first going to look for bare or pinched wires, or foreign material in the area of the neutral safety switch. No telling what could have happened.

From the looks of the diagram, the blue/yellow wire that engages the solenoid travels back to a connector (maybe two?), then straight to the NSS ("start inhibitor"), then up to the ignition switch. With the ignition switch unplugged, the circuit should end there, meaning that somewhere in between it is both grounded, and finding some path to a hot line.

I'm inclined to think it is not the solenoid at this point, but I'll let you know what I find.