. $600/yr in taxes seems like a low amount for a developer to not want to pay in order to acquire an extra plot of land that would enhance their space lol. 

Incidentally, no one is paying us to be extra-special nice to you while we do it.

It literally takes 5-10 minutes to find a replacement, or you could call a local vendor, ask them to find the device for you, and pay what they charge, which is marked up to cover their research time.Keep in mind you can only charge depreciated value.

Also, in my lease for tenants, it states that the tenants are responsible for paying for heat utility.In New York City between the months of October and May, landlords have to have to keep the heat a certain temperature.My question is, am I still responsible for keeping the temperature at a specific number if I placed mini splits in each unit and that my lease says that the tenant(s) are responsible for paying the heat utility?

Principle pay down is not.

Are they overextended, do they pay late on other things? 

Hire a PM before you find you have a non-paying tenant living in your home for 6 months while you beg him to leave while paying his utilities/repairs and probably a relocation fee  I hope you sell, but good luck either way  

How do I pay taxes on my LLC? 

Just to be clear, did M******** ever pay you outside of Zillow or did she mark the rent as "Paid Offline" without you receiving any funds through any method?

All you have to know is how to buy something at the right price so that it pays for itself.