Stubborn Purge Valve codes P0441 / INPA 250

[e39_touring]

So, I'm finally down to 1 lousy error code on the wife's old X (3.0) that I've been prepping to sell for like a year now (it takes that long when you only have time to work on it one afternoon every other month or so). It's the INPA 250 Functional Check TEV or OBD P0441 Incorrect Purge Flow.

Why can't the codes on this car be addressed by the normal/common fixes!?!

So, here's what I've done:

1) Remedied all vacuum leaks, which have alleviated all other codes - had to replace intake boot and dipstick to CCV hose. Smoke tested intake, normal fuel trims, and no related codes leaves me thinking I'm all good on vacuum leaks.

2) Replaced the TEV/purge valve with a new one (BMW original).

3) Swapped gas caps with my other X which has no codes.

4) Ran DTML cycle test in INPA multiple times. Result always a pass with everything intact. Result always a fail with 1mm+ leak if I leave the gas cap loose or disconnect purge hose from TEV/purge valve. This tells me the system is sealed and nothing's blocked between DTML pump, purge valve or tank.

5) Tested the new TEV multiple times - with engine running and valve closed, I feel no vacuum at port that goes to canister. With valve open at 20% or 99% (the two activation values INPA triggers), I feel good vacuum.

Everything seems to check out, and I am perplexed as to why this code keeps popping up after ~30 miles of driving.

My understanding of this TEV code is that when the valve opens, the DME looks for a corresponding change in the O2 sensor readings (lean?). If it doesn't see that, it opens the valve the next time the car's at idle and looks for a change in RPM. If it doesn't see that, then it throws the code and sets the CEL.

My O2 sensors, while probably original (140k mile), seem very responsive and don't cause any other problems/codes. The only other thing I can think of seems a remote possibility - a partial blockage in an EVAP hose or clogged air entry point into the EVAP system.

Does anyone know definitively where the air should enter the EVAP system when that purge valve is open? It's got to be either the filter on the DTML pump, or does the gas cap have a one-way valve that lets air in but not out? I think the only thing I haven't ruled out is some sort of flow restriction.

Has anyone run into a tough-to-eliminate P0441? All my searches lead to stuff I've pretty much ruled out (gas cap, new TEV valve, DTML pump, etc.).