How was the U.S so sure that the Enola Gay would not be shot down, even if the Japanese thought that it was a plane dropping leaflets?

The Japanese did not think it was a plane dropping leaflets; they wouldn't have known what it was doing. Lots of solo B-29s were flying over Japanese cities at that point in the war, doing reconnaissance. The Japanese ability to attack B-29s at that point in the war was pretty limited; what little they had was spread too thin to go after individual planes.

When Oppenheimer made his famous speech after the detonation of the first Atomic Bomb, did the Japanese have access to hearing it?

Oppenheimer did not give a famous speech after the detonation of the first atomic bomb. If you are talking about the "I am become death" bit, he never said that at the time. Oppenheimer gave no speeches about the atomic bomb until after the Japanese surrender.

Why is Poland sometimes accused of starting world war 2? Putin mentioned it in an interview, is there any merit to this claim.

World War II can be considered to have many origin points, but typically the German invasion of Poland from the West, followed immediately by the Soviet invasion of Poland from the East, is usually when we date its European beginnings because this led directly to the British and French declaring war on Germany. The Germans and Soviets, of course, made up fantastical justifications for why their invasions were necessary and justified. I do not know what Putin has said about Poland, but I can imagine it was similarly fantastical, because Putin's Russian state has essentially committed itself to justifying the actions of Stalin, however atrocious. The official Soviet line was that they were not invading Poland, but that they were seeking to preserve its independence after its government fell because of the Nazi invasion. Of course, they did no such thing.

Also, was Russia the real biggest threat to the U.S, more so than Germany? I remember hearing that many Americans believed they defeated the wrong enemy in the war, in reference to Russia being the bigger issue.

American fear of Soviet Communists was profound in this period, and would of course become a bigger fear into the Cold War. One could debate which was the greatest threat to the US homeland. But the Nazis were clearly a massive threat to US interests in Western Europe; they were radically expanding their borders and spheres of influence through the use of force, and would have exterminated democracy in Western Europe if it was up to them. The US alliance with the Soviets was a necessary one in order to stop the Nazis; a classic case of "the enemy of my enemy is my friend." It is hard to imagine making a coherent and cogent case that it would have been better for the US to have aligned itself with the Nazis. It is certainly possible to argue that the US alliance with the Soviets involved the US compromising some of its stated values, but this is the case of many US alliances then and now.