Solution found on the starting problem. I've got a 90 amp alternator because of the external oil cooler fan and the fans on the air conditioning condenser and evaporator. I logged voltages on my trips around the island and found that when the engine was hot and I had the AC on, the battery voltage dropped well under 12 volts at 2200 RPM. So, long trips and the car wouldn't start the next time.
I measured the DC amps on the alternator at full draw and it only hit 52 amps output. Checking the output for alternating current showed over 8 VAC. My thoughts were that a diode or two had failed.
I ordered a 90 amp Compufire alternator as a replacement. It was heavier and had extra vent fans compared to the EMPI one I bought a couple of years ago. They claim to use USA parts for the Compufires. Stuck it in and instantly charged over 13V at idle.
I'll take apart the EMPI and rebuild the diode pack and call it a spare. It was clearly 2nd class quality.
So, I'll reconnect the battery cutoff switch and see if it introduces any problems, but otherwise, I think the partial failure of the alternator was the culprit.