I don't recall any source claiming that Nathan Samuels ever became the UFP president. There is a reference in To Brave the Storm's final chapter to a "Nathan Samuels Room" at the Palais de la Concorde, but I think the novels established that the Palais was the Earth Prime Minister's residence before it became the UFP President's.
Not exactly. Articles
established that the Palais existed as early as Haroun al-Rashid's presidency; Beneath the Raptor's Wing
established that the U.E. Prime Minister maintained an office at the Place de la Concorde, future site of the Palais, in the 2150s, but not that the Palais itself existed.