As you can see, two very knowledgeable officials already have given you different rulings. I don't think there is anything written that will settle this issue and the R will just have to make a judgement.
I'm not saying which one is right becasue I just don't know but I'll make a few pionts that come to mind...
- If A had players being subbed out and none where near the throwin at the time and B had all 5 players guarding the remaining players from A, A would have a very good case for getting the ball back.
- If it was B who had players being subbed, B really earned the ball.
- The primary mistake was by the official administering the throwin...the other, presumably, had the prescribed signal up to stop the action...while the whistle is nice and recommended (and I do it when it might be unclear), I don't believe it is actually required.
- The ball did become live as the throwin was administered.
- The ball was dead because of the hand up.
All this adds up to a big hmmm.