They are not "contaminating" each other as they are spaced out. Cold air is likely being drawn directly back to the return side in the discharge plenum(lower unit)usually the result of a poorly done installation. Drop the cover on the AC and stick your hand up into the plenum. If you feel any cold air at all, it needs to be sealed off/re-taped.
"Since I've only used my hand to gauge delta-T I have no idea what a ~20 degree drop should feel like."
And you will not be able to gauge it.
Unless I'm reading the thermometers incorrectly, I'm seeing around 48 degrees on the discharge side and 85 degrees on the return grill. That's a far cry from the 18-20 Delta a properly sized, charged, and ducted AC should provide. We don't know that the unit is properly charged without knowing the amp draw of the compressor and the outside ambient temp, but I would say the lower discharge temp is likely from some kind of air flow restriction or cold air being drawn back into the evaporator and the high return is because the unit is undersized for the space it's trying to cool and it just can't keep up. At any rate, it's operation is far from acceptable.
As far as air feeling extremely cold from an air restriction, with less air moving less heat transfer is taking place and the air may feel extremely cold at the vent, but if you move away from it, you don't feel much air movement. That's the reason people are fooled into thinking their AC is really pouring out super cold air. It is, but only for a few inches from the vent. If you aren't moving air, you aren't cooling it.
Anyone who understands air conditioning would understand why 2oldman's numbers are not right. He has probably never even used a set of gauges and it would do no good to refer him to a refrigerant pressure/temperature chart because he wouldn't know what he was reading. Some people are best left to their own misconceptions since they aren't really interested in what anyone else has to say. Don't confuse them with facts.
I never understood the rational for having to energize it to cool, as Doug said, the default operating condition is cool. It's been like that on my marine units since I can remember and even when Dometic bought Marine Air and Cruisair, that stayed the same.
If it is a new unit, I would look at getting the dealer to replace it under warranty. The usual cause of a sticking solenoid is a small piece of trash or debris that was introduced into the system when it was built. The valve operates by teflon slides moving from one side to the other and it doesn't take much to hang it up. Not common, but it does happen, and it's likely it will stick on you again. Replacing the reversing valve is labor intensive and requires a moderate amount of skill and experience to do it without burning it up with the torch and most shops are not going to want to do it. I've been changing them for 15 years and my pucker factor still goes up when I have to do one on my marine units.
You can give the valve a light tap on the end with a wooden stick if it sticks again, but noting heavier.
If it is a heat pump, the reversing valve solenoid has to energize for it to run in cool mode. You either have a stuck reversing valve, a defective reversing valve solenoid, or the control board for the unit is defective and not causing the solenoid to energize. It's an easy diagnosis for someone familiar with these units, or you can wait and get every kind of armchair diagnosis here and then take it to a shop.
It is just amazing how so many of these simple questions turn into a pissing match about who is smarter.
And the end result is that after 4 pages of comments, the furnace still works on propane and 12VDC, either from the converter or the battery.
These tech threads just keep getting better and better.