have you calibrated your test for nitrates? Even the "professional" tests have to be calibrated to be accurate.
The other thing
Your regulator should regulate the pressure.
If pressure is building up to a problematic level there may be other problems.
Pulled out my magnifying glass and the FF has 4 "whiskers" 2 face forwards and 2 thinner ones that are set further back. I can not see this
Good deal of investigating. Way to help him out! And friend!