I think both you guys are right:
LW, Michael probably did calm down on the plane ride back for the reasons you said. dt, I figured that Michael had to have phoned Tom as soon as his plane landed in the US (how else would Tom know where to meet him?), and would have told him that Fredo was the traitor. But I like your scenario about Fredo calling Tom in a panic, and Tom, ever the negotiator, already thinking of brokering a deal. That fits nicely.

By the time of the boathouse meeting, Fredo said, "The got Pentangeli, I can tell you that." Well, Michael already knew it because the committee chairman would have had to tell his lawyer, Tom, the identity of the witness to be used against Michael. But I have a suspicion that Fredo's next statement, "That Senate lawyer, Questadt, belongs to Roth," may have been a surprise to Michael. If so, it certainly implied that Fredo was closer to Roth and Ola than he'd let on--and that he'd had info that could have helped his brother if he'd come forward with it sooner. That alone may have cemented Michael's decision to whack Fredo.