Thanks for continuing on with me on this. I am glad Nordyne sells the pan as a part. It looks to me that I can remove the side of the coil case with the humidifier and hopefully access from the front and side will be enough to lift the coil and replace the pan.
I am fairly sure there is a leak in the pan from when I did the test of plugging the drain tube and filling the pan with water. When I did that water immediately began dripping down through the furnace. I will check again for "bridging" sources, but I do think the pan is the problem.
I will try the paper towel/toilet paper test once I have the pan off the coil. With the two attached I do not have a way to dry out and test anything other than the front 1 inch of the pan. The rest is under the coil, attached to the pan with little gap.
I understand your concern about the uninsulated walls sweating. Since the pan itself leaked when I plugged the drain and filled it, that seems to be the problem. This test was done with the front wall open and the ac off for several hours, so I do not think wall condensation is an issue.
I am not sure why the installer necked down from the 4" opening to the 2" pipe, although most all furnace exhausts/intakes I have seen around here are 2".
I will not be able to work on this more today as I need to go and get my daughter who has been on a canoe trip in Canada.
Monday pm I hope to remove the side case and then the pan from the coil. I sure wish there was a way to disconnect/reconnect the copper pipe and pull the whole unit, but I will see if I can get the pan out without disconnecting the refrigerant lines (which would lead to other issues). Once I have the pan off (wishful thinking!) I will look for cracks and test it per your technique.
I will be in touch after I do that work. Thanks, -Matt