Volume of water - minimum - to open thermostat ?

[Refueler]

Looks good to me, when done let air into the intake side so the water can drop out of the leg and maybe tip some antifreeze down that side too.

That's an intended benefit as well ... at present the intake pipe just sits in the bilge .. I have used a length of fencing wire to lift the inboard connection so it is easily got hold off .. but up or down - it does not lose water out of it ...

With the 3 way in place ... Antifreeze can in deed be let into the leg as well as engine pipe.