Problem solved - for now anyway. The "failure" was current leakage to ground through the "line" contact where the screws held the one LED to the heat sink. Looking very carefully I can see slight discoloration around the screw notch in the star mount, where it contacted the conical back face of the screw head.
Today I found some Arctic Silver brand alumina filled epoxy heat conductive cement at Frys. I thoroughly cleaned the heat sink in the contact area, cleaned the back of the LED (using alcohol), and reinstalled the LED using the alumna filled epoxy. After a half hour (it sets up in 5 minutes) I plugged it back in, and only the 6 LEDs ahead of the failed one lit up. So, I removed the attaching screws entirely. Plugged it in again, and it worked fine. That is when I looked for and found the discoloration.
Now I suspect that I will get this failure again, as another LED leaks current at that spot. If so, I will then mount all of the LEDs with the ceramic epoxy and not use screws at all. In the future I won't try to use the screws if I make another one of these.
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