The weirdness of Doctorow's historical fiction

The ending of E.L. Doctorow's The March shows his strengths and his weirdness, all at once. Unlike many of his contemporaries, he does care about shaping the ending of his novels rather than having them end abruptly w/ no real sense of conclusion - something Roth was guilty of even in some of his great works like American Pastoral - and that's probably because ELD ties his fiction not to the arc of the narrative of a character but to the arc of an epoch or event in history - none more so, I think, than The March, which proceeds - the title and the reference to Sherman's advancing army are a metaphor for the narrative process in this instance - toward its inevitable conclusion with Lee's surrender (later, General Johnston's surrender - if the novel is to be trusted a major gaffe on Sherman's part to agree to terms that would not be acceptable to his superiors in the military and in government) and especially toward the Lincoln assassination - material there for a whole other novel, if ELD wanted to go there. He illuminates these historic events through an oscillation between public figures and figures of his own creation, as did Tolstoy, e.g., but I think the narrative interest is heavily weighted toward the historical: great as some of the writing is throughout the novel, would we care as much if he were writing about a fictitious war? I think our interest, mine anyway, depends greatly on thinking about ELD's insight into the character of particular military leaders, the history of this war that took place on our soil at sites that we can easily visit today. And that's where the weirdness comes in: just as we trust the veracity of his research and believe that, yes, he makes all of this history feel real and alive and present, the accurate re-creation and smart creation of fictional characters representing different cultures and contexts (confederate soldiers, union soldiers, an army surgeon, several freed slaves, Southern belles, et al.) he upends his research by involving the key historic figure - Sherman - in an episode that, as far as I can tell, is entirely fictitious - an assassination attempt by a confederate sympathizer and racist posing as a military photographer. I thought the scene was probably real, until I looked it up and found nothing but references to this novel. So why does he do that, play around with the facts at the edges of history? He's not interested in a false history - what if Lincoln has survived the attempt? What if Sherman had been assassinated before the march - rather, he's interested in just bending or refracting the light at the periphery so that we can't quite trust him, so that we are left with the sense of unbalance and uncertainty, that history is malleable, at least in the telling, that it all comes down to - as he notes toward the end of this work - a lot of nouns and verbs.