As far as the return:
That was for overflow should the transfer pump remain on after filling the header tank.
If this happened and there was a vent then it would pump fuel overboard. If there was no vent then it would pressurize the header tank to Facet pressure minus any pressure reduction caused by the sum of the outgoing  and returning fuel from the main rail. I thought even a few psi would be dangerous for the tank.
Steve Izett

Of course – I was thinking in terms of gravity feed from mains to sump as in my case.

Are you suggesting then that if the header has no air leaks then the main pump suction could be used to draw fuel from main wing tank up into header and delete the transfer pump?

Under most conditions it would, but I wouldn’t take that approach.  As Ed says, any air leak would be an issue, and on a hot day at altitude with mogas – don’t tempt vapor lock.

Al