Michael already knew that Frankie Pentangeli was alive and arrangements would have already been in place to bring Frankie's brother, Vincenzo over, when Michael went to talk to Fredo
Yes, Michael knew that Pentangeli was alive before the Fredo boathouse meeting. But, he didn't know that Questadt belonged to Roth until Fredo told him. Knowing that Pentangeli was alive would have raised alarm bells in Michael's mind about what Pentangeli could have testified to. But, knowing that Questadt belonged to Roth would have confirmed that the Senate subcommittee was going to spring a perjury trap on him. I think that's what confirmed Michael's decision to bring Vincenzo to America and to the hearing.
An interesting side question: If Fredo had voluntarily come to Michael,
before he testified, with both pieces of information, would Michael have pleaded the Fifth Amendment instead of lying under oath by denying the subcommittee's charges?
