Audi A3 BMN Engine: Cold Start Problems

Hey guys and girls, I'm having a little problem with my A3 and would like to hear your opinions or experiences about it. Thanks in advance.
Regarding the Audi A3 vehicle, 2.0 TDI, 16V, 170 hp, manufactured in June 2007, with engine code BMN.
"The problem: The vehicle starts perfectly when cold and runs flawlessly without any loss of power or smoke. However, as the temperature increases, there are occasional to severe misfires at idle, as if a cylinder suddenly stops firing. This happens more frequently as the vehicle gets warmer. While driving under load, no abnormalities are noticeable. In partial load conditions, up to 2000 RPM, the misfires are also noticeable while driving. When the engine is turned off in its operating temperature range and then restarted, it takes approximately 20 seconds or longer of cranking to get it running again."
Already renewed and inspected:
"All values within the range of -0.7 to +0.7. The injectors have been thoroughly cleaned and rebuilt, seals have been replaced and the system has been re-calibrated. The injector bore in the cylinder head has been inspected (there were slight signs of wear, but nothing too severe). The wiring harness for the injectors has been replaced, the glow plugs have been replaced, the mixing tube in the cylinder head has been replaced, and the fuel pressure at the tandem pump has been checked (2 bar is desired, but it's 3-3.5 bar at idle). The electric fuel pump has been checked and is producing approximately 0.5 bar, which is within the acceptable range. The tandem pump has been replaced (with a Bosch unit). The problem still persists, and the fuel pressure has been checked again, still only 2 bar at idle. If you pinch the return line with pliers, the fuel pressure increases to approximately 4.5-5 bar, and the misfires at idle disappear, and the vehicle runs perfectly when warm. I suspect that the cylinder head may be damaged. Do you agree with this assessment, or do you have any other ideas about what might be causing the low fuel pressure when the engine is warm? Oh, and the error memory is empty."

Thank you. Best regards.

The 16V engines should have the PD elements bolted on twice and have a conical seal to the cylinder head. Usually, the conical seal in the cylinder head becomes oval. Check the MWB 23.
If you press the return valve and the pressure increases, then the O-rings are defective. However, you've already replaced those. Could it be a mounting error, or was the O-ring damaged?

I re-checked the PDE injectors and inspected the O-rings. The measurement block 023 shows approximately +23 +/- 2 for all cylinders. I strongly suspect a faulty cylinder head. What do you think?