Tore the thing apart. Now everything is under sunlight without mysteries. My guess was not exactly right, which fooled me a bit. Now findings and understandings are:
• The two ports P1/P2 lead to a common chamber with a membrane and a rigid plastic backing actuator plate behind it.
• The triggering pressures of the two switches appear to be (intended to be) identical. (Why two identical switches? Probably for error tolerance.)
• Both switches are arranged symmetrical about the case (left and right) despite the asymmetrical arrangement of the ports.
• For mechanical reasons, the switches behave in a way similar to Schmitt triggers, triggering at pressure levels 700mm and 1100mm water head.
[Edit] I've done further tests double check. Refer to the schematic in the post below.
• The way the control module detects if this invalid condition of the pressure switch is present is like this. When a program starts, DC 12V is sent briefly to the relay allowing mains AC to present on its terminals 3 and 4. If the pressure switch is working, since the circulation motor is now not working so no pressure present, mains voltage will present on terminals 1 and 6, which is detected by the control unit.
(To check this theory, I hooked a DDM across the 12V DC wires, and pulled out one of the two wires from the pressure switch going to the control unit. After pressing the Start/Stop button to start a program, I can see the DMM detecting a brief presence of 12VDC. After a while, the three LEDs start flashing and the machine stops working.)
I hope this information is clear and thorough to anyone in the future who needs to find out if it is faulty.
In my case, the control unit probably detected the faulty condition, when 1/3 and 4/5 are closed, respectively, which is not a valid condition. But I will investigate further.