ARDS: a very overused and misused word.
The scenario presented a significant SpO2 increase by repositioning, O2 and one neb.
There were not many other details such as BP or other factors that can determine Qs/Qt.
I would not recommend "high" levels of PEEP used by EMS unless they are fully capable of achieving an acceptable MAP on the BP with fluids and the appropriate pressors. In an older patient this could be a delicate balance and if the SpO2 remained at 94%, I would hold off on cranking the PEEP knob to "high" levels. Some do not understand how PEEP works and believe the results should be instant and end up demolishing any resuscitative progress. There are other factors to also consider for oxygenation which are also reasons for the fluids and the pressors.