Al, I've seen photo's where there is a filler cap on the return line before the pump, but not remotely located like yours, is that the similar but simpler approach you are explaining?

Only if there is an air expansion volume somewhere is the system.  The idea is to maintain some pressure in the system.  If you have just a pressure cap on system that is completely full of coolant; the pressure will only be there as the coolant heats up and stays constant; the expanded coolant being passed by the cap to the overflow. If the coolant temp goes down, even slightly, the pressure will drop to zero because the coolant contracted and is essentially incompressible.  I you have some air in an expansion tank, the air compresses as the coolant expands, and the air will maintain pressure even if the coolant temp goes down a bit.  Many autos use this approach; an it is likely entirely adequate for aircraft systems as well.

Good idea on having air bleed lines from the Rads High points - what size line and what size fitting did you use? I guess their the two tubes to the centre sides of the filler neck. 

That is all 3/16” line; AN3 fittings.  It only takes a very small line to bleed the air.  After the air is purged, there is a small amount of coolant flowing through there bypassing the radiator; so you want to keep the line small.

Al