Thanks for your help and suggestions. After trying out a number of seat back adjustments and sitting positions we had some success today. Besides the advice posted in this thread, the "Recommended Reading" listing had a number of threads th had some ideas. I find it interesting that this problem with passenger seat detection goes back ten years or more. Anyway, we've found a way for her to light the "passenger seat on" light.
First success was, with the car running, having my wife plop into the seat and sit with legs out, not flat on the floor. Also seemed to work after having her lower the seat. We drove a couple of miles to the bank. On the way, I pulled over and stopped, turned the car off and restarted. Damn "off" light came on. Then I had her raise her arms and push down from the ceiling to put more force on the seat. That worked and once the on light came on the "off" stayed off and "on" stayed on until we parked at the bank. On the way back I stopped and restarted several times. In each case she needed to use a ceiling push to get the "on" on. Once lit, it seemed to stay li (acted normal).
At least this seems like a solution to get the airbag armed. We'll see if there are tweaks to positions that can be made so that it is automatic or maybe things will loosen up. We were going to see the dealer tomorrow (with her sitting in the rear seat) but we'll now cancel that visit and see what happens.