Include in lease that you do these things-- and be sure to follow through if you include them in lease, so you don't get some tenant's house on fire and trying to sue you for not replacing smoke detector batteries.

We have moved to 10 year battery models, so tenants can't forget to change the batteries.

I would suggest removing all four, and temporarily replace with basic, inexpensive, battery operated units.

The batteries end up costing more than the device though  

Everything sounds so good until battery dies.

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.