The floor in question at CH remains a total mistery to me as well. I agree with you CHRISinUSA,that the double drawing room is above the dinning room/library. and your correct in thinking the picture i posted (the princes private sitting room) is above the morning room, The room is very similar to the morning room in size, and it has the one large window facing The Mall and three slimmer windows facing west.
Now looking at the pictures, it looks like the prince uses his private sitting room like a study as well (with the large desk playing centre stage), a bit like what The Queen does in BP so i would be surprised if he has a separate study. I thought the round window room, above the portico could be the prince's bedroom, Its a very private room as the portico hides it away and the prince often has tall flowers growing above it, but you mentioned something I’ve never thought about, which is a hallway window, which actually sounds more realistic otherwise it would be a very dark corridor without any natural light. If that’s the case, The Prince's private sitting room would be isolated as its a corner room.
an older picture of that room when the Queen Mother had it, which I’ve posted below, and when the Queen used it, shows there are two doors, one i assume leads into the corridor and another facing towards the double drawing room, so perhaps the prince's bedroom faces west and is above the library, just before you get to the double drawing room (only a suggestion).
from the picture of the Duchess's sitting room, I would have thought one of the doors (either right or more likely the left) leads into the duchess's bedroom, so that would mean it was above the Garden room. after all these rooms are the private rooms so it’s likely they directly lead off to their “masters” bedroom.
The sitting room, in the Queen Mother's day (The two door's)

the sitting room in Princess Elizbaeth's (The Queen) day

You can see from that picture that the ceiling has never changed.