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.