Last night I reloaded my arguments and tried again. Because I was not sure what notes the earlier agent had put in my file, I explained my situation to the promotions agent in some detail. He instantly agreed with me, said the ATL/ABQ/BZN/ATL itinerary was indeed a circle trip, worked just fine in the computer with the AmEx companion certificate, and saw no problem with the reservation. I asked him to just note that in the reservation, which he said he would. I was then put on hold. Then a long wait ensued and I thought, uh oh. After 5-10 minutes, he came back and said that someone in customer care would explain to me why the itinerary would not work. I spoke with a supervisor who said that a circle trip was the same thing as an open jaw, and my reservation was not an open jaw trip (certificate says open jaws/circle trips permitted). I said then why did they even need to say circle trips are permitted (not to mention the conceptual problem of an open jaw and a circle being synonymous). She would not budge.
After receiving the posted replies today, I called promotions again this time armed with the contract of carriage. Once again the first agent agreed but chked with a supervisor. This time the supervisor agreed with me, said there was no problem with the itinerary qualifying for the companion certificate, and personally called the city ticket office to confirm they are open all day on Good Friday. She noted the file, said she knew the workers at the ticket office, and said to have them call her if any problem. This supervisor is a 27 year Delta employee who was not worried about a higher-up sanctioning her. She stated that circle trips almost always include a stopover and since the certificate permitted circles, she believed the circle authorization overrode the stopover prohibition that otherwise existed with my itinerary. At one point she laughed and said, "Don't try to figure it out."
A final observation. She was a supervisor who looked for a way to make the certificate work for the customer. The other supervisors seemed so afraid they would be guilty of a violation they were not willing to go out on a limb on a confusing issue. Thanks to all of you for the input that kept me going and actually made the companion certificate have some value. Price for two=765 over the 7/4 week (dodging the blackout dates on certificate and dealing with the LUT prohibition).