Sub metering is installing a meter on each unit.

The rental needed a lot of work, I installed central air/heat replaced the water and gas lines and put new windows in the house.

Friend got it with this at a vacation home.Forget the billing and fix the problem:First take better pictures of the meter and the connection for evidence.Get an electrical contractor to come out, disconnect the illegal connection and install a security box with a lock so there's no access to the power  connects.Write a letter, send it Certified to the neighbor with copies of the tampering that this is criminal fraud and theft and if it occurs again, they will be prosecuted - - clearly note a copy has been sent to the Utility Company.You might research the owner of the property and send a copy to them also (in case it might be rented).You didn't start this but need to control it.

Even if they don't,  also look at your new remodel and ask if you want amateurs moving in appliances that may or may not be functioning (leaks etc) properly or installed properly.

I am currently tearing out my brick fireplace to install french doors, among several other things.

You need a license to advertise a property, no option can give you the right to advertise or show a property and not be considered a listing agreement, such agreements may cause the option to be an installment sale as well. 

Should I install a sprinkler system or let it be? 

However, like an RV a Park Model cannot be permitted as an "Additional Dwelling Unit" (ADU) aka "Granny Flat" and therefore is not allowed to be permitted or permanently installed on a real estate lot or parcel.Unlike their big brother the HUD Manufactured Home, Park Models are built under the ANSI code and cannot exceed 400 square feet of living space.

Typically we just make sure the grey box is install properly but I personally have not had them installed or requested to be doing bank work or flips.

What is needed to complete:-Additional painting and touchup painting of walls, trim, and blinds-Repairs to oak wood flooring, plus sanding, sealing, and clear-coating the entire floor-Laying marble tile around a fireplace-Installing a sink in a granite countertop, including cutting the holes, etc.