Hi,
I am not sure that actually solves my curiousity either.
but that is not to say it isn't working in many cases either.
Water takes the easiest path so to me if the pump is chugging away at idle putting through enough water to comfortably flow through the thermostat with little effort or pressure then why would the water travel to the other end of the head to escape. At speed I can understand pressure would force the water out the heater outlet, but at idle perhaps not.
Using the bottom hose or the lower radiator inlet then you will have either Venturi or Syphoning affect respectively drawing the water from the heater outlet almost regardless of what the pump is doing.
Once you have all this connected I would be interested to see if you can connect up a clear hose between your copper tube and the header tank and see if there is a change in the water flow once the thermostat opens.
Terry