The fact is, NO residential AC will cool a space more than about 15 degrees cooler than ambient.

It's basically a thick sheet of firm rubbery material you put down under the flooring that helps prevent not only foot traffic noises but also ambient noises from passing between floors. 

; and HVAC systems (limited actual range of output in relation to ambient).

I'm reading online that basement apartments can cost less to heat because they are below the frost line and additionally the ambient heat from the upstairs unit.

I'm not sure if they make the low ambient units in a multi-head unit though? 

(...taking 60F water, making it 212F degrees, to produce 72F degrees) If you installed 'staple-up' radiant floor tubes under the floors of your units (assuming you have access to them via a basement or crawl space) you would only need to heat your floor tubes to about 80F or 90F to produce that 72F ambient air temperature.

I modelled one of our student rooming houses (1400 ft^2) and every degree over 20C when the ambient outdoor temperature is -10C (warm winter day or night) costs ~$15/month.  

I contacted some mitigation companies today and it looks like $3-4K to have a "ambient" treatment performed where they heat all units to 130+ degrees for 5 hours and then spray with chemicals. 

If you have pipes run in insulated cavities or runs that are mostly beneath ground level, I wouldn't be too worried as the ambient temperature wont drop as drastically as the exterior. 

The vacant unit may be colder than the occupied unit but just the ambient heat from the other unit is enough to keep pipes from freezing, especially when coupled with that running furnace in the basement.