It is pretty obvious from eye witnesses, the Zapruder film, basic ballistics, etc., that there were two shooters - one in front, one behind.
Not to put too fine of a point on it, but JFK’s brains could not be sprayed all over the trunk from a shot from behind.
There is also the sworn testimony of one of Oswald’s coworkers that he was giving Oswald’s change for the coffee machine (I think) when the shooting actually happened. Imagine being Oswald, realizing what just happened, rushing back to his work area to find his rifle from home in front of the window. He knew he was being set up. He ran. He probably assumed they were going to shoot him on sight.
The side of JFK’s head exploded. Blood and brains absolutely could have sprayed all over the car from a shot from behind.
Oswald brought the rifle with him to work and lied that the package was curtain rods. There were people in Dealey Plaza who saw him standing in the window of the Texas School Book Depository shortly before the assassination. He was never some innocent patsy. Remember, he murdered officer JD Tippet too