OK still trying to find the gremlin. I had re-wired the hour meter as the mechanic recommended, and it is running fine. Tach and everything else still works fine. The kill switch was working after that, but now it no longer works at all. This seems to have developed (or progressed) on it's own, not as a result of my changes.

When I use the kill switch now, it blows the fuse (labeled #6 in the circuit diagram). This is a 55A fuse that costs $11 and troubleshooting would get expensive, so I replaced it with a 50A breaker from Blue Seas which I found in WM for about $30. I'm now looking for a short, so I measured the impedance at (#13 in diagram) the solenoid valve aka Kill solenoid (with wire connected) at 0.1 OHM. 0.1 ohm is a short -- not a dead short, but close. My next step is to remove the positive terminal from the head of the kill solenoid and measure the solenoid separately from the circuit that leads to it. If the solenoid is shorted I have found the culprit, but if the circuit is shorted I'm not sure where to look.

FYI -- I already replaced the ground relay (#5) on a hunch, and no change -- so that's not the source of the fault. (The diode cable has also been replaced.)