ORIGINAL: RangerJoe
The reason Japan attacked the US was the stopping of the oil exports to Japan which the DEI also followed. The reason for the ban on oil exports was the occupation of French Indochina but since France would be an ally of Japan then that would not be a problem for the US. So there would be no ban on oil exports to Japan and hence no reason for the attack on the US at Pearl Harbor nor anywhere else.
The US reaction to Japanese occupation of French Indochina was not to protect French colonial possessions. It was an expansion of the war against China, and that required the reaction made.
As far is it being known about Babyn Yar, it was known to those people who had an interest in such events as well as certain people in power. As far as every single person in the entire world knowing about such an event, you are correct in that not every single person would have known about it. But the people in power and/or interested in such power in the US knew about such things but at that time, there was only so much that could be done about them. Heck, the Communists and Socialists in the USA were protesting assisting the Allies until after the 22nd of June, 1941 . . .
Still, just rumors. Certainty didn't arrive till tanks rolled into the death camps.