Oli the pnly problem I see with your assesment is that it was Neri, and only Neri in whom Michael confided that he wanted Fredo killed. Further, in the scene in whch Michael dresses Tom Hagen down, Rocco has a little smirk onhis face. I believe Rocco was keeping tabs on Tom's doings, his mistress job offers etc., and was passing this on to Mike, nor so much on Mike's orders but with the sense that he was Michael's eyes and ears. He also was in on the killing of the hooker/compromise Geary operation. All this amounts to a great deal more than being another Luca or a bodyguard.

Shrewd as Michael was, I believe he set up a kind of rivalry between Tom and Neri, with each thinking he was the No. 2 man, but never being entirely sure about it.