This is just about the most extensively discussed question on this board (other than "who killed tha Tahoe shooters?").

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.

Some people here believe that Michael brought Frankie's brother Vincenzo to the hearing to shame Frankie: he couldn't break omerta after seeing his old-school brother watching him incredulously. Others believe that it was an implicit threat: if Frankie went ahead with his testimony, his brother (and, perhaps other family Frankie had in Sicily) would be harmed. It's possible that both viewpoints are correct.