Fat ones! Bosch 0280150785 are oranges, 315 cc/min. Bosch 0280155759 are reds, 315 cc/min. Your 97 turbo is a 2.3 HPT and shipped from the factory with EV1 type 0280150785, which are orange. That's why they are in there. The EV6 type 0280155759 may appear orange in some photos, but they are red. The...

The blue-plugged sensor should be your IAT. How does that connector look?

Have you checked the FP regulator? Should be down low behind the engine.

The piece on the fuel rail is the damper, not the pressure regulator. The pressure regulator is down low behind the engine, like I mentioned. https://turbochuckcom.ipage.com/webimages/projects/97%20Volvo%20855/Engine%20Rebuild/FPR.jpg There is no temp sensor in the air inlet. One of those sensors is...

The IAT is referred to as the "Outer Air Temperature Sensor" for the ECU, and yes you have the general understanding correct of the function of those sensors. That sensor may not throw a code, but if it's registering -40* there is most likely a problem with it. If you unplug it and test it...

... Last question, is there a special way to check FP regulator's health ? One thing you can look at is the vacuum line that connects to the FPR. If the vacuum line smells very strongly of fuel, then the FPR is damaged internally and leaking fuel into the vacuum system. This is not a complete test ...