The Magic Bullet Theory was not put forth by conspiracists.
It was suggested by then-Warren Commission counsel Arlen Specter. Specter was hardly a tool of the "JFK was killed by a conspiracy" crowd.
As for the witnesses, there was one who had military training (and I believe was a combat veteran) who said emphatically that he could be shot coming from behind him and over him. The guy in the railroad tower (behind the picket fence) saw several individuals back there with weapons. He saw gun smoke and heard shots from there. The individuals left right after the shooting.
As for the bullet marks, let me clarify. It was two. In fact, afterwards investigators cut away the part of the curb that was struck. They never said "Yes, a bullet hit here" nor did they say "No, a bullet didn't strike here". Your attempt to say "Was it 2 or 1 or none" doesn't change this.
You're free to discredit the evidence of course. The evidence is less than perfect. But using a rational analysis of all the evidence combined, it becomes clear that there was a strong probability that there were multiple shooters.
Until someone comes forward (or new evidence comes out into the open) that can reasonably say that there was one shooter and it was Lee Harvey Oswald, there's no compelling reason to avoid believing that there were multiple shooters.
You will notice that in my posts I have been using the words "strong probability" and strong likelihood" in describing why there were multiple shooters. Unlike you, I'm not prepared to rule out any possibility. You, on the other hand, have clearly ruled out the possibility of multiple shooters.
Like I said above, we're gonna have to agree to disagree on this one - until the day (hopefully) where new evidence comes forth.