What does the ending of the Plot Against America reveal?

The ending of Roth's alternative-history novel reveals, or at least suggests, different "facts" or truths which can be said to have validity in different regions of reality, so to speak.
On the most concrete level, the revelation that Philip Roth, as narrator of the novel, has to process is the explanation that the Lindbergh presidency and its pro-German, isolationist stance are the result of the Germans' having blackmailed Lindbergh by kidnapping his son, the famous Lindbergh baby who was allegedly killed years before. After Lindbergh himself has disappeared, there is no way anyone can verify this story. It seems, on the one hand, to exonerate Lindbergh himself, implying that the aviator's whole program of antisemitism wasn't the result of his actual beliefs, but was forced upon him. One doesn't know if the fact that, in Roth's novel, the country has eagerly gone along with it is the result of genuine hatred of the Jews, or is simply a manifestation of the herd mentality. The Americans regard Lindbergh as an idol, a hero for his solo transatlantic flight and also an object of enormous sympathy because of the kidnapping of his child. Because his authority is on such a high plane, the US population is willing to follow him no matter how irrational and bigoted his policies are.
But there is a "higher" truth, above these cold facts and assumptions, that animates the last part of the book. After the whole episode of alternative history has reached its close and, so to speak, expired, actual history takes over. Roosevelt is re-elected President, the Japanese do attack, and the US enters World War II on the side of the Allies. In this restoration of normalcy, Roth suggests that the Lindbergh presidency has just been an aberration, something that could never be sustained. The implication is that American values (our intended values, that is, whether or not we have always been successful in practicing them) will triumph in the end. It is a hopeful revelation in an otherwise very dark story.