In an effort to explain everything outside the scope of directorial license (or oversight) I would posit that Fredo knew Queatadt belonged to Roth because he saw him in Havana with the Roth entorage. Alrthough he was kept pretty much in the dark it would not have been a big deal for Fredo to know that Roth owned the senate counsel, and frankly he would have probably never connected the dots until Michael was hauled before the committee.