It is true that a Marksman badge is the lowest badge the USMC gives, but they mean it when they call it "Marksman". A USMC-designated Marksman is not a dolt who can't hit a barn from the inside; the Marines emphasize rifle marksmanship more than any other branch, and take quite a bit of pride in it; it's not a consolation prize or an attendance award. Oswald was 100% a loser, but incompetent he was not.
Anyone whom the US Marine Corps qualifies as a "Marksman" is certainly good enough to hit a 8" target 1 for 3 at 80 yards with a 4x scope from a benchrest, no? And the third hit was barely a hit (almost missed high and right). I'm sure he practiced quite a bit with that rifle, both live and dry firing; I suspect most failed reenactments of the shot involve shooters who have NOT practiced for weeks with the rifle beforehand.
And reacquiring a target moving directly away from the shooter at 11 mph in 2 or 3 seconds using a low-power scope in full daylight against an asphalt background is NOT hard. Plenty of hunters also manage to reacquire and shoot followup shots at animals in two or three seconds, using much harder kicking calibers against much more difficult backgrounds. Shoot, I am not a great rifle shot and I can recover from recoil and transition from one IPSC 5-zone to another in 1/2 to 3/4 of a second, and an IPSC 5-zone seen through a 1x optic at 15 yards is comparable in apparent size to a head at 80 yards seen through a 4x scope.
I do not personally believe that Oswald was a lone wolf, but I do believe that his being the lone shooter is not inconsistent with 3 shots in the time given.