The only way for Roth to "engineer" the near fatal attempt on Pentangeli's life is that he had only the cop who happened to walk past the joint on his payroll. In this scenario, the Rosato Brothers were probably in on it as well, otherwise Roth would have made a stupidious risky move, because he could have lost his allies in New York in a firefight with the police. Or they could have turned on him instead if they would find out that Roth had betrayed them. The other agents who arrived at the scene for backup could not have been involved in Roth's plan, otherwise they wouldn't have fired like cowboys.

In the end, whatever the plan was, it turned out to be quite a mess.