High velocity rifle bullets can do very odd things. My 2nd deer - shot with a 100 grain, .25 caliber Nosler ballistic tip, launched at about 2900 fps, hit deer 200 yards away, perfectly behind shoulder. The bullet catches a rib, turns a 90 degree turn and goes out the bottom of the belly!
As for Oswald, it's been proven he could have done it.
More importantly, it isn't credible that some organization with considerable resources conspired to kill JFK. One, too much risk of screwing up the hit - Oswald gets caught before the shot, sings, and spills the beans. Second, a shot at a moving vehicle is a low percentage shot, something that only a lone individual with limited opportunity and resources would try.
Think about it - why take the risk of getting caught? If JFK is dead, and the conspiracy is discovered, whatever cause or organization responsible would suffer the unbridled wrath of all America.
Why not just arrange for one of JFK's skeletons to come out? Arrange for one of his mistresses to be discovered, or die suscipiously? Kennedy is ruined, his legacy about as bad as Clinton's is now. Kill him, he's a martyr, especially if his murderers are then discovered.