Roth and the Rosatos intended for Frankie to be dead. The cop breaking up the attempted murder was pure happenstance. There is no way--repeat, no way--Roth could have arranged the cop's entry with split-second precision to break up the murder in order to get Frankie to turn against Michael.
The "Michael Corleone says hello" had to be an artistic whim of Coppola's. Danny Aiello admits it was an ad lib, so to leave those words in the film was a director's decision. For what purpose? Consider that a film is an artist's interpretation. There are many things that create dramatic effect and imagery; music, lighting, sound effects, make-up AND dialog. Those four words add a little punch to the scene and it can raise viewer curiousity. We tend to forget how a scene can affect us when viewed ONCE. Most of us here have viewed the scene hundreds of times. The initial impact has been covered over and distorted by years of analysis.
