Both of these claims are untrue. Brennan was the only one who ever claimed to see Oswald on the 6th floor and he only did so several days later after initially failing to identify him.
There were witnesses who claimed to see a person with a gun on or near the knoll (Jean Hill, Rosemary Willis, Ed Hoffman).
Was this person seen to be aiming the weapon by any chance? Pretty sure Brennan saw a rifle being aimed downrange at the exact time of the shooting sequence... and by the way Euins saw the trigger being pulled at the exact time of the second shot.
Brennan chose not to ID Oswald out of fear for the safety of his family. Buell Frazier left Dallas out of fear for his family's safety. Are you now going to once again laugh at the concept of a man putting the safety of his family ahead of everything else in life?