The house was unoccupied and thermostat set on 75.

Personally, I would look into possibly adding another thermostat for the downstairs tenant then split the bill 50/50 with both tenants.

They have individual temp control/thermostat too.

They will probably work directly with the manufacturer to install the heating system which is controlled room by room on thermostats. 

I highly doubt the thermostat will be the make or break point for the house.

The current owner installed 3 new HVACS: one HVAC for 3 units on the 3rd floor, one HVAC For 3 units on the 2nd floor and one HVAC for a larger unit on the ground floor.So there is one thermostat for each of the above floors and they are simply set to 70 degrees year round.

This allowed me to verify that the door being closed in the common area, with the furnace running, would be enough to prevent the pipes from freezing.However, the Tenant has turned the thermostat all the way down, but still on, and I believe is still using the space heaters to supplement. 

Has anyone tried to use smart thermostats to track individual units heat consumption.

However, there is only 1 thermostat (control panel) for the whole building.

& @Ron Fletcher ThanksBryan - the three reasons at the bottom of this article may help: https://www.coolray.com/help-guides/why-do-my-air-...1) Leaving thermostat fan set to "ON"2) Many guests or pets.