I declined unlocking the thermostat but offered to lower it to 68°F.

I have heard people on these forums talk about using the Nest thermostat to set a maximum low + high temperature, to keep guests from blasting the a/c or accidentally bumping the heat up to 98. 

Ring doorbell or similar camera to check on who comes in and out, and smart thermostat so you can change the temp before guests arrive or when they leave.

Locks, thermostats, lights, cameras, home automation, supply delivery,  anything you can think of that will allow you to monitor your house and keep records of activity. 5.

Everything Ken said plus plaster walls, no insulation, higher electric bills, walls sweating when its humid in florida and the resident leaves front door open, damp smelly houses, Pealing paint, wood rot, old windows, crawl space under the home, bdrm and closet doors that don't close properly, electric locks that the old front door doesn't like as the door is not aligned correctly, Cyrstal knobs that fall off, old thermostats with 2 lines but my new thermostat needs 3 lines, critters in the fireplace....

Secondary loan programs require a permanently installed, thermostatically controlled heating source. 

In the coldest days in the winter the heat in the units never actually reaches what the thermostat is set to even set to say 65degrees.

Considering this, how would you be able to handle the potential objection of the bottom unit tenant complaining about AC/heating since it's likely that the main unit tenant will have access to the thermostat?

I remember a tenant who decided it was easier to open a window than lower the thermostat in winter. 

First time stopped to work in 4-28-2024 and disconnected from thermostat without any idea why.