You can't be perfect, there is always some sort of turbulence around the static pressure source, the dual static source is useful to counted sideslip but does it counter it perfectly ?
No measurement is perfect and the closer to perfection, the greater the cost. However, a major proportion of the flight test dollars go into PEC workups and the resulting AFM charts will be as good as one can get. I suggest, in terms of the thread questions, we can take the PEC to be near as close to correct for all practical purposes. Introduce some sideslip and it goes a bit out the window. For most aircraft, in most operations, sideslip will be minimal and unintentional so all usually is OK.
I don't know the specific term in english but during flight test they use some sort of pole/rod to measure air data very far away from the aircraft.
Probably you are thinking of the various air data test boom and trailing cone kits. Some links at
Air data boom - Wikipedia
Design, Testing, and Calibration of a Custom Air Data Boom to Obtain Flight Data For the UTSI Cessna-T210J (N33UT) (tennessee.edu)
What Is The Shuttlecock That Hangs Off The Tail Of Test Planes? (simpleflying.com)
These bits of kit keep the source away from the bulk of the aircraft airflow interference so the crew can determine the errors inherent in the aircraft's pitot static system by reference to the free stream measurements taken with the test kits.
The pressure vs altitude follows approximately an exponential law,
Actually, the ISA model (and any other of which I'm aware), is exponential, as is the derivative to figure the rate (on a given day, the numbers will be a bit off ISA, as you infer). For SL, the figure is around 27 ft/hPa (rather than 30) and the graph in my first post's link is the model for the derivative with altitude. Not sure that I concur with your calculation but, no matter.
a real Australian pilot is being prosecuted for ‘low flying’.
Then I suggest that the pilot in question secures the services of one of the lawyers who both specialises in aviation and has a significant flying background (presuming that he/she has not done so already). There are a few of those folk around (JM and GP - both long term mates of mine - come to mind, for starters) and they would then engage an appropriate engineering expert (and, again, that is an easy ask) for the courtroom punch up. If the ins and outs of the thread are relevant to the case, I would have thought it would be a relatively easy exercise to have it thrown out of court without an excessive waste of time.
A side issue might be if the other side can involve your friend in strict liability matters along the way ? Part 91.267(4) would be a point to consider ...
And when someone sees something that they think is small but is in fact much larger, it appears much closer.
.. and, hence, slower. Well can I remember the first B747 I saw coming into land donkeys years ago. I was sure that it was about to stall and make the headlines that evening ...