The tenants use the heat like CRAZY in this place....Can I legally put a lockbox around the thermostat and set it at a fixed temp?

Like I say, I totally picked that at random with about 15 minutes searching on realtor.com.Better areas would be Phoenix NorthCentral & NorthEast, Mesa North & East, Chandler, Gilbert, Tempe, and Scottsdale.

A friend of mine lived in Tempe.

Ultimately, it's going to depend on your investment goals and strategy.For example, if I'm working with someone in the Sacramento market, it's probably going to be a Bay Area investor who'd like to invest in a market with good appreciation and rent growth, but is still located in California, and is lower priced than Bay Area where most deals don't make sense on paper... too pricey.But if it's Phoenix, Scottsdale or Tempe we're talking about, it might be someone looking to AirBnb / VRBO a small condo in a hot part of town.

My main concerns are splitting bills, mail, and AC temp (only 1 AC unit).

If you mandate that the A/C be on from June-September that is fine, but if it was set to "my home" temperatures it would never turn on because of the temp would never drop below.

Tenant left the hose connected to the spigot over the winter (live in Midwest where freezing temps occur during Winter). 

I have started looking at Tempe, Mesa , Chandler and Scottsdale. 

These have a special jumper that you can move, set the max temp, then move the jumper back.

Doing this will provide each room with their own comfort options, but each room will need their own remote to control the air handle temp.