First, lookup Fair Housing Regs and maybe there's a State version too (there is in Calif).Try the Nolo Press books for Landlords and its parallel for Tenants.

At this point to not make any demands of repairs, it will seem retaliatory and will make them want to press the issue to get a new tenant.  

I've had success sourcing deals for other investors through mailers, cold-calling.

I know a plumber that says," it's actually a cold water heater, because hot water doesn't need to be heated."

Is this a good idea or should I press for gas heating?

There's nothing wrong per se with cold calling - - you might just catch the owner when they've made a decision to sell but not yet signed an agent contract. 

First the tenants were improperly using the trash compactor to “see what would happen” if they threw a bike or cinder block in it and pressed the start button.

Greetings everyone I've noticed there aren't many 4plexes on the MLS , so I wanted to know where can I find a List of 4family homes in Oakland county Michigan , thinking of cold calling owners to make them offers,any help would be appreciated .

This was Virginia, so not very cold, but right around New Year's we had some pretty cold weather.

I was thinking of either sending handwritten letters to local dev firms, cold calling or emailing.