It is sometimes said that Stalin knew more about the atomic bomb through his myriad of spies than Truman did. This is no doubt true. FDR was arrogant on this score in my opinion. In his dying days he could have briefed Truman in on everything, but he didn't, wanting to hold on to power a little longer. It really wasn't fair to Truman.
Not sure if I agree with what you wrote. Much of what I've read lately suggests that FDR may not have been all there during the last campaign, and until he died..We'll never know how lucky we are that the Dem power brokers dumped Wallace from the ticket and replaced him with HST,,