Wow, what an interesting guy. I can offer some speculation which might help you, but no solid citations.
It looks like your grandfather concentrated on Europe. With his OSS background, I would guess that he transferred to some sort of air unit that supported American OSS (Office of Strategic Services) and British SOE (Special Operations Executive) operations in Europe.
That probably meant he did a lot of nightime air drops of personnel and supplies into German-occupied Europe, probably at night. This was an extremely dangerous sort of job, as the Germans had good radar, antiaircraft, and night-fighters. In those days, night flying was more of an art than a science.
One type of insertion involved dropping “Jedburgh” teams (possibly derived from the Scottish term “Jedburgh justice,” which implies punishing someone for a crime and then trying them afterward, which is what these guys could expect from the Germans) into France. These teams consisted of an American OSS man, a British SOE man, and a Free French man, and they had to be precision-dropped in areas where they could be assured of meeting sympathetic locals–they wore uniforms in the hope that they would not be executed as spies if captured. (The Spy Book, pp. 299, 408-409). There were at least 93 of these missions flown. This type of mission strikes me as the kind which would most likely net your grandfather awards from our British friends.
If he was a Jedburgh pilot, it would also explain why he warranted a Gurkha bodyguard, obviously a courtesy extended by the Brits. However, I cannot identify what air unit performed these types of flights. I wouldn’t be suprised if this unit was quietly tucked away among the 8th Air Force or Bomber Command, or possibly even the 1st Airborne Army for security purposes.
I’d also guess that the plane he never checked out on was the P-38. It was a twin-engine fighter which was extremely expensive, and some say it was difficult to fly well. In the first half of WWII the planes and their pilots were husbanded like some sort of treasure, because it was the only allied plane at the time which had a good enough range and performance to cover bombers most of the way to their targets inside Germany.
Anyway, I hope I haven’t steered you down the wrong path. Good luck in your search, and please let us know what you find out!