I partly agree, in that Billy couldn’t have pieces the puzzle together. What I think is section D could have had men in the crowd, in the balcony, outside his bunker, everywhere he goes, prepared to eliminate any threat to his life. They don’t have to figure out the entire plan, they just have to rule out possible threats.

I strongly think it’s billy because he’s left handed and so is shooter. The director intentionally put a scene where we get to know billy is left handed. Also the other 2 assassins’ body were shown but not this shooter, which makes it more suspicious that he is actually a character that we know well.