I don't know what is so hard to figure out about the roles of Neri and Rocco. Initaially, Neri was selected by Michael to be at Michael's command to carry out special orders. The novel makes it clear that he was also a body guard, trying to stay physically close to Michael to protect him. It's quite clear to me that Neri's role evolved. As I posted elsewhere, call Neri an underboss, a consigliere, or just a capo. By any name he was the person that Michael trusted most. Rocco was not a rent a cop. Why would you state that? He was a capo and carried out Capo functions. That was the Corleone hierarchy during Michael's tenure. There's nothing mysterious or conspiratorial about it.