Why exactly did Frankie cooperate with the FBI?
Was it because he believed Michael was behind the attack?
Or did they have anything to put pressure on him? If so, why didn't they blackmail him earlier?
And if they had no charges against him, he could have tried to run away.

Its says they wanted hik for murderx drugsx and extortion. An yes he thought Mike tried to have him killed and he survived.

"Michael Corleone says hello."

Law enforcement (FBI, local and state) probably had plenty of charges that they could have leveled against Pentangeli. But, as a caporegime, he probably had insulated himself in the same way Vito did--giving orders to one person at a time so that law enforcement could not produce a corroborating witness against him. I'm guessing that Cicci's defection provided law enforcement with that corroborating witness. And, Michael's "betrayal" gave Frankie further impetus to cooperate. Never underestimate "revenge" as a motivation in the Mafia.

Hagen says they had a pile of dirt on him.

The combination of Michael's betrayal and the chance at saving his own skin was enough to motivate Frankie to turn traitor.

Tom's line always struck me as funny. "They'd already had him on possession, bookmaking, murder 1, and a lot more."

Once you throw in Murder 1, what else do they really need? What could the "lot more" be?

Also, what does Tom mean by "possession?" Possession of drugs?

Was old-school Frankie really likely to be personally involved with drugs? He is unhappy with the Rosato's emphasis on the "junk dope." It seems unlikely he would "possess" drugs.

W, when prosecuting someone for a felony, especially an organized crime someone, a multiplicity of charges would be very useful to a prosecutor. They provide leverage for a plea bargain. Of course, there is no guarantee that Frankie would be convicted of murder. The reamining charges provide a prosecutorial safety net for the state.

As far as possession goes, state and federal statutes define possession as having effective control of something illegal regardless of whether one physically possesses it. The lot more could be racketeering, extortion, jury tampering, intimidation, bribery, stolen goods, etc.

"Possession" could have meant possession of an unlicensed gun, but I think it was just sloppy writing.

Whe Tom said they "had" him on all those charges, he probably meant that all those charges were in his dossier. But, if they could prove any or all--and had witnesses willing to talk-- they would have arrested him and brought him to trial.

I think "Michael Corleone says hello" is one of the most confusing, yet crucial lines in the history of film. Ironically, it was ad-libbed. I always thought that the line of dialog is why Frankie took the stand against Michael. He thought that it was Michael who was ordering him to be killed. And like Turnbull always says, it was also meant for the bartender, to make him think that it was Michael behind the murder.

IN GFII, Michael tells Frankie that Clemenza promised the Rosatos three territories in the Bronx. Frankie states that Clemenza promised them nothing.

Who was right? How would each know? Why would they disagree?

Good question, Oli.

I think that Clemenza proved his loyalty to Michael, and after the move to Nevada, he kept Clem on a long leash. Maybe Clem mentioned something about the Rosatos--perhaps some job they did for him that he said he would reward them for. But, I can't see Clemenza directly involving Michael in the details of what would be, for Michael, a minor operational matter occurring far away in NY.

I think Roth pushed the Rosato affair on Michael for his own purpose--to deliberately stir conflict between Michael and Frankie, and to make his allies (Rosatos) stronger in NY. I posted a long time ago that Michael should have been suspicious of Roth pressing the Rosatos' claims. But I think Michael's greed for Roth's Havana holdings dulled his suspicions. And, if he did question Roth, ol' Hyman might have fixed him with the gimlet eye and said, "Michael, they helped me once and I owe them a favor. Do yourself a favor and go along--it'll be good for our deal."

Maybe Frankie was in possession of drugs. In fact maybe he was high when he broke Omerta.

They charged him with "failure to prepare something for Michael Corleone when he visited defendant's home."

That was said by Carmine Rosato's brother. The Corleone Family wouldnt kill him for no reason.

I know why Frank killed himself : Because, like in Roman times, if someone betrayed the emperor they were given the choice of killing themselves and in this way saved their families from the shame. Also, their families would be taken care of, unlike what would happen to their families if the shame was put onto them.
Tom gave Frank this choice when he betrayed Michael by testifying against him at the Senate meetings on Organized Crime. Tom told Frank that his family would be taken care of after he was gone.