Show me where I said that he would call off the assassination. You miss the point. He simply wouldn't have placed himself in that position as a gunman (lone or otherwise) with minutes to go and people everywhere who could have testified that they saw him going up minutes before the shooting started.
Lee Oswald was in the 1st floor lunchroom when Jarman and Norman walked by, at about 12:26 ? 12:27 .... Jarman testified that they took the west elevator and arrived on the 5th floor at about 12:28.
Lee was in that 1st floor lunchroom when they walked by, and they took the elevator to the 5th floor, so Lee would have had to have left the lunchroom after they were aboard the elevator...and then climb the stairs to the 6th floor. He simply couldn't have been on the 6th floor at 12:30.