I love your analysis of the story.
Perhaps, Jack is too close to Nicky. Randall, Kate, and Kevin have family ties but are strangers. They can look more objectively at the circumstances while having ties through Jack. Maybe that is direction the story is headed. Maybe, they can do what Jack could not without dragging his family into the muck.
An impartial second set of eyes and ears are often needed to unravel the mess that families get themselves into.