I would still make sure you get the cold temperature rated version that don't have to back up heat generation in freezing temps. 

We have a Airbnb outside of Detroit, what is your opinion of what temperature the house should be set at before the renters get there?

Never had an issue but am told (the house is professionally managed) that maintenance on it is intensive between checking chemicals in the water, maintaining the water levels with heavy use (and water temperature by extension since it takes 24 hours to heat up to normal operating temp after refilling), filters, hair, other cleaning etc etc etc

The one major downside of having the central air is that your tenants may disagree about what is the most optimal temperature and making them separate would probably be best long-term.

A freezer over-freezing probably would have had you adjusting the temperature settings and monitoring it for a few months.

I scheduled my initial inspection in the temperatures were projected to be in the 90s, so I showed up in shorts and flip-flops.

Here in Georgia we've not hit the problem but the leases I have do contain a clause that if the temperatures go below freezing the tenants are required to allow faucets to "drip" during the night to prevent pipes freezing. 

The attic should be drawing enough air in from the soffits to keep the temperatures relatively cool up there.

It took me years to be decent at it, there are a few little tricks to it (the angle of the cut, taping the tube to the gun, temperature of water).

Does anyone have recommendations on automatic space heaters that turn on at a specific temperature?