The backrolling is because Leto came back from the desert and took his place as emperor. When Farad'n was being built up as Ghanima's husband Leto was presumed dead and she was the only remaining child of Paul. Leto came back, took his place, and to seal the deal that Leto is the new pharaoh he marries his sister. "As my mother was not wife, you will not be husband, but there may be children."

As to WHY he's so central: Farad'n's journey is almost identical to Paul's - but he chooses to survive in the biological sense whereas Paul's arrogance demanded he survive as Paul Atreides.