Aiello admits he ad-libbed the famous line, and Francis Coppola, for some reason (probably inadvertence), permitted the ad-lib to remain in the film, to the eternal bafflement of Godfather fans. But it’s also possible that Coppola, the most careful of directors, allowed it to remain because it fit the plot, even though Carmine intended to kill Frankie all along. “Michael Corleone says hello” was intended not for Frankie—but for Richie, the bartender, whose ginmill was being used to set up Frankie.
It’s obvious that Richie is a “civilian,” not a Made Man, and he’s nervous as hell about his bar being used for a murder (“Carmine, NO, not HERE!” he screams after the cop enters and Rosato draws his gun). Carmine knows that Richie might be squeezed by the cops investigating Frankie’s murder. Richie would be too fearful of Carmine to identify him as the killer. Still, as a civilian, Richie is not bound by the code of omerta. So Carmine hands Richie something he can give the cops so that Richie can get off the hook: “The murderers said, ‘Michael Corleone says hello.’ ” That line would set the police after Michael, and would be picked up by the press-- another nail into the coffin of Michael Corleone’s “legitimacy.” Clever Roth!
As for "someone very close to us," it's the number-one stumper on these boards. No one knows for sure who killed the Tahoe shooters. Some people here (including me) believe Fredo opened the drapes to allow the shooters to machine-gun Michael and Kay's bedroom. But who killed them????