I find your statement about the A-4 development team difficult to believe. An American report published during the war stated "larger rockets are known to exist." A post-war British report stated that long-range rockets had been built. Based on my own studies, they were doing a number of things right up to the end. The supersonic DFS 346 was another project that was approaching the finish line. The Horten Ho XVIII was under construction at Kahla in April, 1945. (Source: Nurfluegel by Reimar Horten and Peter Selinger.)
Ed
Ed