Your petrol engine will either have a mass air flow (MAF) sensor to measure air flow, or it will have manifold absolute pressure (MAP) sensor to measure inlet manifold pressure. It will not have both as there is no need. The inlet manifold is actually under vacuum, but the ECM doesn't understand vacuum so it interpretes it as negative pressure instead. With the engine not running the MAP sensor reading should be around 100KPa. This is atmospheric pressure, and will depend on your altitude above sea level. The higher you're located the lower the reading within a few KPa. With the engine idling and the throttle closed the MAP sensor reading should be around 40KPa. This figure will be higher than this if you have a manifold air leak. If you snap open the throttle the reading should initially go back to around 100KPa because there will be no pressure difference either side of the throttle valve. When the throttle is released the MAP sensor reading should fall below 40KPa before settling back at around 40KPa. This is because manifold pressure drops as the engine revs down with a closed throttle, and the pressure difference either side of the throttle valve is greater. So the scantool measuring the MAP sensor is very useful.

Short term fuel trim is the O2 sensor reporting to the ECM the state of the fuel/air mixture. Long term fuel trim is the the ECM's corrective response to STFT. So if the LTFT reading is 10% for example it means the engine is running lean and the ECM is adding fuel by 10% to maintain the correct fuel/air ratio to achieve Lambda 1.

If the ECM has to correct too much a fault code relating to the O2 sensor will eventually be generated because that's where the ECM is receiving the information from, and not necessarily because the component is at fault.

You need a garage who understands the system, and not one who's just going to throw parts at it.

Any other tests done that you "forgot" to mention?

Have you had the engine coolant temperature sensor read in live data?