The idea of having the shed partially in ground is an excellent one!!!!
My pump house for my well is mostly submerged and it never freezes or gets hot!!!!
How does your swamp cooler actually cool the air? It evaporates the water. When water evaporates it uses energy from the surrounding air/water and causes the surrounding air/water to release this energy and cool! This sounds like your best approach for cooling. Have a thermal switch turn on a fan when the tank temperature exceeds a set limit. The fan blows air over the water causing it to cool.
For heat... I would directly heat the water not the surrounding air. It will be much more efficient to heat the water than to heat the surrounding air so that the surrounding air heats the water. Your swamp cooler principal comes into play here to. The less water evaporates from the tank the less heat you will have to put into the water. Cover your tanks to reduce evaporation in the winter!