Great post.
How many accidents do you hear about when the FO didn't speak up? Plenty.
I guess they are the auditors of the flight. Not in command but making sure the commander has got it right. As you say the ultimate filter.
When the **** hits the fan the FO can make or break the operation.
This argument is ultimately one for the neuroscientists of the world. They will explain in simple terms how experience effects one's ability to process information. I saw a documentary once on it, fascinating stuff.