Regarding nose up and arse up and suction that you mentioned, in my limited mind, gravity must be a contributing factor how a fluid acts.
That's just my thoughts by the way.
A little like a closed central heating system in the house, leak on a downstairs radiator if you have a boiler in the loft.
Disregard my ramblings, I'm no plumber but it seems to make sense to me.
Regards.
John
Your completely right in your thoughts but there is a but. Diesel is an incompressible fluid, well almost but suppose it is. If you block the output from your tandem pump and drive it will lock up to the point it lifts the veins and diesel flows back. Actually there is a relief valve that protects it but ignore that for this discussion. The pump will remained locked. Release the pressure and the pump goes nowhere; nothing to compress, nothing to release. Pressure comes and goes with input mechanical pressure.
So the pump draws fuel from the tank and the pressure at the inlet to the pump is negative. Switch off and it is still negative so it could suck air in via a leak to flow back; only if you have a leak. No leak no flow back; incompressible fluid. Think about it like this, stand on a brick, does it go anywhere, no but it is under pressure. Step of it and it is not under pressure. Does it go anywhere no. Take a brick between two hands and stretch it, negative pressure, does it go anywhere, no. Diesel is the same unless there is a leak.
Switch off and you have negative pressure at the pump and unfortunately a leak.
Car arse up less negative pressure and less air gets in.
Car arse down more negative pressure and more air gets in.
All of that said I was resisting this post before I had checked for;
Return valves in the return feed at the fuel sender
Capillary drain at the tandem pump return
Mixing of return and fresh fuel in the fuel filter.
Other things to understand too but I hope this is not disrespectful nor wrong and helps.