This is a question I've been pondering for years and still can't quite come with anything. I don't agree that the policeman was part of the plot to disrupt the murder of Petangeli. It was the cops interference that resulted in the following shoot out leading to multiple deaths. So that just make any sense.
The night before, Fredo received a late night call from Johnny Ola asking if Petangeli was really going to make a deal with the Rosado brothers, or if Petangeli was bringing his "boys." Fredo didn't know and hung up. I was thinking maybe the "Michael Corleone says hello" line was said just to let Frank know the Rosado brothers were tipped off by the Corleones. Sort of a taunt. Not so much ordering the murder. Best I can come up with.
The whole point of Roth wanting to get Michael out of the way was to install Fredo as the new Don. That would have made it real easy for Roth to get the Corleons involved with his Havana dealings, which Michael was reluctant to do, and too smart to jump right in. But even that would have failed as we see Michael appoint Tom Hagan, not Fredo, temporary Don until it was all sorted out. Smart man that Mikie.