The facts are that Oswald was a above average shot. The range was between 60 to 90 yards, firing three rounds in four to eight seconds, on a slowly moving target. U.S. ammunition companies tended to load undersized bullets for the Carano's 6.5mm caliber. Using surplus military ammunition the Carcano is just as accurate as the bolt action service rifles of its era. Using a bolt action Oswald only had to work the bolt twice. So he was a trained shooter, using a scoped rifle, with a sling, and properly sized bullets.
For Oswald to have acted alone is entirely possible and based on all the evidence, probable.
Showed what one motivated Marine and his rifle can do.
And yet, from a rested position, shooting at a stationary target about 100' (not yards), using the exact same rifle, he missed General Walker.
Oswald had the lowest level of USMC marksmanship qualification. The Marksman badge.