One of many factors which led to the assasination maybe, but unlikely (I would think) to be the sole reason.
As you said, nobody here (in this thread) said that Marina's actions were the sole reason.
And nobody who knows anything about the actual shooting believes it was done in "world class time and accuracy." Oswald completely missed with his first shot; took between 6-8 seconds to fire the next two shots, and missed with his second. It took him three shots and, in my opinion, more than eight seconds to kill JFK.
Hardly world class shooting. If it was world class shooting JFK would have been "taken out" with the first shot.