One of those inherited tenants opened it (which anyone can do- changing battery, etc.) and crossed the wires so that the boiler would go continuously.

I agree with @Ronald Rohde, it sounds like simple battery Schlage keypad deadbolt locks would be a good fit. 

Just put four batteries in, throw a piece of chocolate biscuit into the big 10 inch tunnel like structure and the idea was when the mouse runs in it gets Zapped by 8000 volts.

Things may work today , but the RE market could tank and you lose that opportunity , but my crystal ball is out of battery and I cant tell the future right now . 

I know typically battery backup is used when the grid goes out but not sure if I'm able to pull from battery to avoid tripping any circuit breakers in the main house. 

One nice feature with Minut is that its battery powered, which means that you can place it _anywhere_, on a wall, ceiling, etc.

Are you able to change batteries on technology like Ring devices, Smart Locks,smoke detectors, etc.?

You can do silly repairs yourself… like figuring out the batteries in the thermostat are why the AC won’t turn on; that it was a tripped circuit breaker even though they swear they checked them.  

Mortgage: $2,634Utilities: $300Misc: $300Total: $3,234The misc. is things like toilet paper, paper towels, coffee, sugar, linens, batteries for remotes, laundry detergent, soap, etc., etc.

And if there's a dog, charge them increased rent, cause you're gonna need it to recondition the yard after they're gone.