IIRC the witness statement that he heard Gray banging his head on the wall was in the news shorty after the news broke that gray had died in police custody.
It was. He disavowed it within hours of making it. During the meeting in question he reportedly said both his first and second statements were correct, and he saw no contradiction between them.
Yes, it was.
Don’t know why the prosecution thought they could get away with covering it up.