You said there is no evidence of a second shooter yet I've yet to see evidence from any witness that stated they saw Oswald with a rifle in the TSBD or in the 6th floor window with a rifle. In a court of law in the United States first hand witnesses to the fact are first and foremost in any trial. They form the basis of most prosecutions and defences. Circumstantial and forensic evidence are used in conjunction with witnesses to the fact.
What nonsense. Eyewitnesses are the least reliable form of evidence. Convictions can be and often are obtained solely on forensic evidence. There was a witness who saw Oswald fire the final shot but his ID of Oswald is probably the least compelling piece of evidence against Oswald. The case can be made entirely on the forensic evidence. That evidence includes finger and palm prints, fiber evidence, ballistic matching, medical evidence, film evidence, and a paper trail proving Oswald ordered and was the owner of the murder weapon. There isn't a scrap of any of the above types of evidence that indicates there was a second shooter. All the forensic evidence points to Oswald alone.
Nell and John Connally were first hand witnesses to the fact, they were in the limo and if they both state that John Connally was hit two seconds after Kennedy was first struck then that is what we have to go with. If you don't then the constitution of the United States isn't worth the paper it was written on. That would be akin to suggesting that first hand witnesses are not trust worthy in ANY trial.
I don't know where you got your understanding of constitutional law but nowhere in the Constitution does it say eyewitness accounts are required to obtain a conviction nor does it say eyewitness accounts should be accepted at face value. I've served on four juries, two civil and two criminal. Each time the judge instructed us to weigh the credibility of the witnesses based on a number of factors. I was on a murder trial where we had both eyewitness and forensic evidence. The only disagreement among the jurors was whether the element of premeditation had been proven. I was one of two who believed it had not and we prevailed. We convicted the accused of the lesser charge. I also sat on a trial in a check forgery case. The prosecution presented no eyewitnesses to the crime yet the forensic evidence was so convincing we convicted the guy in less than an hour. Eyewitnesses are not end all, be all nor should they be.
There are five video's available of five railroad workers who testified on film that they saw gunsmoke and heard shots fired from the picket fence. I'll post them later. None of them were allowed to testify before the Warren commission. Show me one video or piece of evidence from anyone who saw Oswald with a rifle in Dealey Plaza.
Correction. They saw SMOKE. Modern gun smoke does not billow. It dissipates almost immediately and if it had been fired from behind the fence, it wouldn't have made it into the clear for anybody on the overpass to see it. Lots of people had different ideas about where the sounds indicated the shots came from. Some said all three shots came from the GK and some said all three came from the direction of the TSBD. The three spent shells in the sniper's nest tells us which group got it right.