when something really shocking and unexpected happens, I sometimes freeze, and can't act immediately.
This. I can't imagine, but in much, much less fraught situations my reaction was "danger get away scary things happening go go go." And I wasn't the target -- I think I've actually reacted better when I was personally threatened than when I was a bystander.
This is part of the whisteblower's dilemma, where if you don't react instantly, you're stained too. I definitely wish McQuery had done more, immediately or even days later. But he didn't know about the other allegations, while Schultz did. It's possible he thought, "It's my word against Sandusky's. One of us is a beloved figure with powerful friends, and the other one is me."
It also seems like he must have approached someone else eventually, right? I don't know how this works... What led the investigation to him? It seems like nobody else who knew about that night would be inclined to bring it up.