You are correct in your thoughts that there is only a positive lead for the coil inside the armored section. AFAIK there isn't another circuit that turns on and off with the key as well as staying on in the 'crank' position. My PV has a replaced ignition switch anyhow, so when I first got it it probably had the reverse of most cars - a cut cable but still using the original coil.

In very general terms a weak old coil will still work fine at idle. It is at full throttle with lots of pressure in the cylinders that the spark has a harder time bridging the gap. Sparking the mixture isn't quite on a pass/fail grading system, there is a range of states between a full miss and a full power ignition. My PV's engine would sort of 'waver' at full throttle. Power would build as the throttle would open, but would eventually stop increasing and seem to sputter slightly. Replacing the original coil with it's thready thin blue spark with a Mallory Voltmaster and Crane XR700 electronic ignition fixed it quite nicely, with no other changes the engine started to pull strongly at full throttle all through the RPM range. I kept thinking it was fuel related prior to that.

Note that when you remove the old coil you will have a sizable hole inthe firewall to cover somehow.