[n86121]

It looks like we finally got it: Like most complex things, simple once understood.

Turned out to be just the check valve beneath the fuel pump, against the rear firewall.
Facing DOWN from the fuel pump into the airframe,
it connects excess vapor/fuel return from the pump back into manifold/fuel tanks.

Since the tanks are higher up, hydraulic pressure from the tanks above comes UP the check valve, which mounts vertically.

With the check valve disconnected above the check valve, it would stay dry.
With no fuel coming back UP from the manifold/tanks.
The check valve holds, because hydraulic pressure coming UP the valve from the fuel manifold/tanks would push the valve up/closed.

With check valve disconnected below the check valve ...it would also remain dry,
because nothing coming down from the fuel pump at idle cutoff.

But fill both sides with fuel, so pressure is balanced above and below the check valve,
and the small valve inside would seep just enough pressure back into the fuel pump to also get past the flow divider, then out to the cylinders, then out the drains.

...Now I just need to cleanup the goat I slaughtered trying to divine the answer.