Although the point of who invented, (or came upon), the all moving tailplane has always been a contentious subject I have a feeling that this is one of those subjects that their will never be a definative answer to. The German wartime researchers were working on jet and rocket aircraft through many companies and following many paths but nearly all these projects were designed to operate at high subsonic speeds. I guess this is because there really was no practical need to attempt to push on to supersonic speeds because of the opposition they were facing. They were, after all designing warplanes to reverse a very dire situation. However, this is not to say that research may have concluded that an all-moving tailplane would not have existed in records captured by US forces at the end of the war. The most advanced German supersonic project was the DFS346 but this fell into the hands of the Russians. There's no doubt that Miles did fly their all-moving tailplane and that the research was sent to the US by the Attlee government. Evidence seems to indicate that Bell may have stumbled on the same answer following practical problems with the X1 project. My theory, for what it's worth, is that in fact a number of people hit on the solution and...a). The Germans probably hit on the solution at about the same time as Miles but ran out of time to persue it. b) The British had their project ruined by the usual lack of money/idiot political interference. But c) The US had the funds and the drive to push through their project.
Spiney