What is the temperature differential between the cold air return, and the vents?

The vast majority of homes here are built without A/C both because energy is much more expensive here and because temperatures rarely get above 80 degrees.

It takes more energy to go back to the desired temperature that way.  

A properly operating AC should not freeze up just because the unit can't reach the set temperature.

I think there’s an issue with the A/C or maybe a blockage in the ducts because the a/c is running nonstop but won’t cool below 75 degrees (it’s set on 68 with the fan running). its been hard to regulate the temperature in the house for awhile but more intermittently.

They assumed a family of 4 in the local area where incoming water temperatures varied significantly by season.  

Oddly enough, the temperature swings can be almost 50 degrees from day to night.

Over the course of the year, the temperature typically varies from 43°F to 73°F and is rarely below 36°F or above 82°F.  

I'm curious to know what's the right thing to do in this situation...I've explained to the Bottom tenant he has the luxury of controlling the temperature of the house.

Currently, the owner pays all utilities.Clearly, it's not ideal to only allow one tenant to control the temperature (which is what the current set up is).