Also, it didn't get up to a hot enough temperature even though it was brand-new.I love steam saunas, but it's easier to just use the one at the fitness center.

Thats more like a temperature check on how optimistic everyone feels right now.Any long term prognosis can only rely on supply and demand projections.

I have no idea what the outside temperatures are (ie it is really hot out or cooler) and is she doing things to help keep the house cool? 

You could charge a flat fee on utilities with a clause that states if bills exceed $X, then you split based on square footage.I've done all of the above and it has worked out.Regarding the thermostat, you may want to put in a mini split for the JADU in case your tenants like one way but you like the temperature another.

80 is an extremely high temperature to keep a home at.

Instead of "ceiling fan" in the description highlight "the cool breeze provided by the energy efficient ceiling fan creates the perfect temperature for getting a good night's rest". 

In other words, guests will only be able to control the AC between your locked min/max temperatures, but they would not be able to change to heat mode.

"Hey, the AC isn't holding temperature and it's making a funny noise outside".  

During this heat wave the temperature on 2nd floor has been in the upper 70s and even hit 80.

I've told him multiple times that he needs to maintain a reasonable temperature in the house but he continues to ignore me.