I don't even need to get that fancy, the supply line to the IP is clear.
where that would help is to potentially turn an air leak into a fuel leak to be able to locate it.
There's a thermostatic tee on the fuel return line that returns under-temp fuel to the filter, and hot fuel to the tank - it's a common air leak source. I swapped that tee between the two TDIs and the issue stayed with the wagon. It could be another fuel fitting, could be a mildly cracked line, could be those pump seals. I think it's unlikely to be a single injector leaking down since even with one or two injectors primed it would start quick and rough as opposed to no-start then rough then smooth.
What confuses me is the intermittent nature.
It's (almost?) always fine after less than an hour. 4 hours is enough to give issues. Being warmer when leaving work at lunch today still gave me a hard start. But it starts easily once in a while. It's probably 50/50 or maybe a little worse when it sits for more than a couple hours.
Aside from at the airport it hasn't failed to start.