I don't think that the killer saying "the buffer told me to kill Frankie and say Michael says hello" would have any more legal effect against Michael than if the killer said just "the buffer told me to kill Frankie."

Coppola was in a terrific bind with GF2. Every minute of the flashback material took time away from the main Michael-Fredo storyline, but the De Niro scenes are the only thing in the movie that make Michael a tragic figure, instead of a run-of-the-mill monster. The movie is more melodrama than mystery, so I think Coppolla made the right choices, even if we have to connect a lot of the dots ourselves.