During the Hoover years, the FBI gathered a lot of information from illegal wiretaps and other electronic surveillance. They couldn't use it in court, but they'd use it to confront a criminal or potential witness and make them dance to their tune. So, yes, it's entirely possible the FBI used Valachi to reveal what they couldn't reveal in a Senate hearing without revealing their illegal surveillance.

Much as I admired Charles Bronson, "Valachi Papers" wasn't one of his best vehicles. It was a cheap production, and it shows: in once scene, set in the early Thirties, Valachi is driving a '34 Ford at night--and is passed by a '70's-vintage Ford station wagon. In a later scene, he's driving a '40 Buick, and a modern Checker cab passes by. At the opening of the famous Apalachin NY Mob conference, the title says, "November 1957"--and the cops show up in '59 Fords.