The key to understanding the JFK assassination comes from "reading" the witness testimony given to the Warren Commission by people that were there that day. For instance.
About 15 minutes before the Presidential motorcade came by the was a man named Arnold Rowland who saw a man in a white low cut t shirt holding a rifle with a scope on it standing in the sixth floor window among the stacked up boxes. He was wearing blue pants. The day Oswald was arrested he had a white low cut t shirt and blue pants.
Right before the motorcade came by two women Carol Walters and miss Walton testified that they saw a man in a white low cut t shirt on the third floor holding a rifle with no scope on it.
So if you 'add' these three testimonies together It shows that Oswald planted the M Carcano on the sixth floor and came down to the third floor and shot JFK with a different rifle.
I think the key is understanding that human eyewitness testimony is unreliable and needs to be treated as such. Rowlands story changed several times, he included details in his testimony which were false, his wife who was with him didn't back up his account and when she was asked about if she could rely on everything her husband said she replied that he was prone to exaggerate..
Carolyn Walther was stood with a friend when she says she saw the person on the lower floor. She didn't mention it to her friend, nor to anyone on her return to work.
The human brain is not an accurate recorder and we can imagine we see all sorts of things and can 'remember' things which never happened.