So interesting! The way Billy told Kanbei about The Shimoyama incident and Napoleon and 9/11 I had the theory every event in our history and the manga's history happened all the same, without changes. But this JFK arc had me thinking it could be changed somehow. Seems that's not the case though.
Billy did say, JFK would've taken them to the moon sooner. Kevin even drew as much. If he became the sacrifice instead of Kennedy like in his comic, the Americans would've found the moon bat sooner and WW3 would've broken out. (Somehow)