Well after reading those, I feel that Coppola just wanted a character the audience could identify with to be the assasain. If some random button had done it, we wouldn't had none it was really the Corleones. But when Rocco kills him it's clear who was behind the murder.

I don't know about Rocco being the traitor. Seconds after the hit happens buttons/guards are on the scene who are probably in Rocco's regime. He would have had to been pretty fast and covert to pull off killing the guys and getting back to infrom Michael. We never really find out who killed them, just that Fredo had something to do with Roth's guys getting into the compound.