By now, so many people have duplicated the actual shots fired by LHO, with the same cheap-o equipment, that your proposition simply doesn't hold water. Superior shot needed? Well, a good shot, anyway. AS always, good luck certainly is no handicap.
In re the FBI testing Oswald's rifle: in the time elapsed between the deed and the test, having been in police custody, and having been handled (or even mis-handled) by all and sundry, it is no surprise that a low-end scope would need to be re-zeroed. However, in their conclusions, the FBI allowed as how the LHO-single shooter theory was quite plausible.
Speaking of 6.5mm, I still own a 6.5X 55 Swede, which, a bit hotter than the 6.5 X 52, and the 6.5 X 54 and uses spitzer bullets rather than round-nosers, is a 200m "tack-driver" on the range without a scope. Very easy recoil, very easy target re-acquisition. The next step in combat rifles for the USA may well involve a 6.5 or 6.8 MM configuration. The Dutch, The Greeks, The Italians, The Swedes, The Norwegians, all came up with the 6.5 MM as their military answer to the range, penetration, lethality, accuracy compromise. BTW. The penetration achievable with a long, slim 6.5mm FMJ can be quite something, sometimes blowing through a whitetail and coming out the other side ... or close to it ... looking pretty good. I for one, have never had that much of a problem with the "Magic Bullet," especially since new research shows how Connally and JFK were lined up when struck.
I am reasonably satisfied that LHO was the sole shooter. The real question is who directed him. Again, I vote Fidel... who coopted the Mob ... and who had KGB help.
While I think the Carcano is a bit of a clunker, it certainly is capable of doing the job. I have no arguments with those who say the equipment was capable.
Now as far as those saying the shot was easy, I flat out disagree, at least for a mediocre shot with a scope which is not sighted in properly.
As I have said before, I think Oswald probably was the lone shooter and was really lucky or unlucky depending on your point of view. The fact that it could be done is enough tho the fact that it is a bit unlikely adds to the skepticism of many.
I do have questions as to who put Oswald up to it. I have an open mind but think it is likely someone else was involved.