Just how unlikely is it? Is it simply inconceivable, or just difficult?
The shots have never been recreated. Most scholars and pundits work off that basis and then jump off to more elaborate theories. It was definately possible for Oswald to have made the shots, though you've got to give a guy with admittadly poor marksmanship all the benefit of the doubt you possibly can. I shy away from saying that it's simply inconcievable, because this is one of those, "Well, anything's possible..." situations. In my opinion, however, Oswald didn't fire all the shots himself.
In the aftermath of a major event like that, people start recalling all kinds of things. Multiple people witness UFO abductions too. But it takes more than that to dismiss the professional forensics analysis done at the time, does it not?
Multiple witnesses recalled both a police officer in the area (who incidentially told them to leave despite the general crowd's closer proximity to the motorcade anyway) as well as a puff of smoke. I concur that mass hallucination or fabrication is very possible, but am loath to chalk the eyewitness accounts to fame-seekers.
The fact that many people had motivations does not mean that many people actually fired shots.
True, although I still feel Lee Harvey Oswald was insufficiently angry with Kennedy to have committed the action independantly.