want accurate mechanical temperature switch

Normally closed solenoid valve, or "power pill" valve. Don't know if they still make the latter. Regardless, probably going to need a vent, which can be passive but a potential PITA.

A normally open solenoid valve to drain the pipes and a vacuum breaker outside faucet should do it.

Here's a valve that opens below 35F.

-jsw

No, I'm getting the kind of valve that is open when there's no power.

So, you do already have a thermostat that is sufficiently accurate to do what you need.

Add a second thermostat in parallel with the first one. Hide it or lock it so the +5 degree one can't be modified.

They are supposed to drain the water as well.

That depends a lot on: How long do they stay? How cold does it get? What keeps the pipes from freezing when it's occupied?

Use a wind-up timer that they have to wind up every day and push the reset button that restarts the pump if they forgot. Use a random Klixon sensor at a temp high enough that tolerances don't matter.

If the timer runs out and the temp is below 40F or the power is out, shut off the pump, drain the system. Maybe even disconnect the adjustable thermostat and use the 5F one.

This assumes that there's no downside to drain the system and disable the pump when it's hot outside and unoccupied. The devil is in the details.

Change the locks and tell them that when:

it will again be made available.

The visa card switch might work, too. ;)

But I'd set it up so cottage heaters were on 24 hr timers which had to be set each day they were there. And water pumps were in a heated area which automatically engaged to prevent freezing in the first place. That would require backup batteries and perhaps internet alarms, if not solar chargers.