There is nothing to suggest that Oswald was not the shooter. He had occasion, motive, means, and opportunity. An independent analysis by the International Wound Ballistics Association determined that the wounds were entirely consistent with the geometry and sequence of events. The Carcano carbine was entirely adequate to the task.

I have also read, however, of a second person in the building, with his own rifle. If Oswald was seduced into performing the deed, it is entirely feasible that the parties behind it would have established a backup plan to assure success, while still pinning it on Oswald. In the event, the backup wasn't needed. Oswald's shot was true. The mysterious second man simply vanished during the hubbub.

Regarding gunshot residue tests: "A negative result on someone could mean they were near it but not close enough for gunshot residue to land on them, or it can mean that the gunshot residue deposited on them wore off.[a] Gunshot residue can also be removed from surfaces by washing, wiping, or brushing it off, so a negative result cannot fully rule out a gun was not fired by the tested object or area. There are many possible scenarios that could explain the results, but they all require more evidence to be proven." [Wikipedia]

And what would be so difficult about wearing latex gloves while doing the shooting, and disposing of them afterward?