I decided to try what you suggested tonight -- but still no luck. I removed the relay and respliced the large wire, but it still will not crank. In fact, before splicing, I turned the key to the start position and touched the bare ends of the wire togther to see if I was getting a spark, but there was none, so power is apparently being interrupted to the ignition switch.
Later this morning I will look under the dash for another relay and purple wires as you suggested, as well as check for some some loose or broken red wires from the junction block to the ignition switch, which was suggested in another thread here.
Since lights, turn signals, windows, locks, radio, sunroof, fuel pumps, horn, etc. are all behaving normally, there is obviously some other ignition-related connection that I need to find.
I removed quite a bit of wire and cable during my initial cleanup, but until I removed the lower dash, nothing impacted the car's behavior.
Thanks again for your feedback.