John you have done a very thorough job investigating, cleaning and documenting the problem. You know your cooling unit is fine because it cools on AC. Your flue is clean with a new baffle and you adjusted the propane pressure. The only thing left is the orifice and the burner tube. John Wilson already spoke about the orifice. The last thing to check is the slots in the burner tube. Is it the original one? I know on mine after about 17 years and multiple cleanings with a wire brush the slots became wide allowing more air to the propane mixture. Does the flame flicker at all? Try covering one of the slots with a screw driver tip does the flame get any brighter?