Chris;

1. Open upstream of FIC wouldn't affect Idle (it would mean drawing unfiltered air only)...an opening to atmosphere downstream of FIC would allow drawing in false (unfiltered) air and certainly raise Idle.

2. Fuelrail pressure is THE most important thing for D-Jetronic! ...and "fuel is draining down and pulling air into the rail through the injectors" I don't think flow (of ANYTHING...fuel or air) is possible in that direction...think of the pressures...under No conditions is pressure EVER higher in intake than rail...don't overthink this!

Point of note: By careful calling it closed loop...that suggests FI control is closed loop ...only later systems do that with the O2 sensor(s). D-Jet is strictly open loop fuel control...because it doesn't sniff the exhaust, controller has no way of knowing what is really happening!

As far as the fuel supply system, it would be better to call it a circulating system...FuPu and FPReg bring rail to a constant pressure, FI Controller and Injectors control consumption and the volume not used during operation is circulated back to tank...but you knew that...

Cheers