The air conditioning is fully functioning again. Of all the possible causes of icing-up: thermostat, compressor, obstructed airflow, low coolant, it took me all this time to discover the most obvious one. We were gone for months. Somehow, I just forgot to change the filter when we got back. The filter was dirty and restricting airflow. I put a new filter in. Proper airflow restored. We can sleep at any temperature we choose again. We didn’t have to change our ways after-all. I feel like a bit of a bonehead though. Should I take any solace in the fact that it didn’t occur to any of the trained professionals to check the filter? I don’t think so. They probably thought it was too obvious to even ask the question.