I've said this before, so please bear with me:
We think of Geary as dumb because he thinks with his dick. But he was a politician, after all, and he had some guile. I think he showed it in his two-faced "defense" of Michael Corleone at the Senate hearing:
He said "these hearings on the Mafia are no reflection on the great Italian people, and it would be a shame to let a few rotten apples spoil the whole bunch." Who was he referring to as "Mafia," if not Michael? Michael was the bad apple. Geary was being cleber in making it look like a defense.
His questioning of Cicci seemed helpful to Michael. He asked Cicci if there were always "buffers" twixt him and Michael, and Cicci replied, "No, I never talked to him." That question and answer made Michael believe that the committee had no higher-ranking witness than Cicci. And since Cicci said he never got an order from Michael directly, Michael was emboldened to lie under oath. As we know, it was a trap: the committee was secretly holding Frankie. My point is that Geary, as a member of the committee, had to know about Frankie, and so I believe he deliberately led Cicci's questioning to help trap Michael into perjuring himself.