My $0.02 worth

As far as flow during idle... The system is a closed system.  So if
there is no air in the system and it remains closed then the force of
the falling coolant will suck other coolant up the wing header (inlet
pipe) kind of like a siphon.  The water pump is adding just enough force
to keep everything moving through the engine at idle.  Another way to
view the problem is like a closed solar heating system where natural
convection keeps the fluid moving.

If you open the system up or add a significant amount of air to the
system you cause the water pump to do more work and cause pressure
differential throughout the system or lower the temp to a max of 212 deg
F but you will loose coolant very fast through steam or cause steam
pockets in the engine that will stop the flow altogether.

Jere