How badly do you think Beauchamp mangled the story of what happened that night in Greeley's when he turned it into a dime novel? He was pretty clearly already writing it up in his head as soon as he was reasonably sure he'd convinced Munny not to shoot him. I just wonder, with his uncertainty about the order in which people died, and his curiosity about the Spencer rifle, whether the retelling would have in any way resembled the actual events?
Would he write the story with himself as an active participant, or from a (supposed) detached point of view, merely the person relating events as they were told to him? I'd think the fact he was present would add to the story, but perhaps readers wouldn't approve of the author placing himself in the story. Plus, Beauchamp would probably leave out the part where he started gibbering the moment Munny pointed a firearm at him. Would it become clear to him that Ned didn't actually kill that first cowboy, and if so, would he mention the circumstances of Ned's death? That he was whipped to death as part of an interrogation? Then again, audiences in the late 19th Century might enjoy such things. Would he leave Munny's line about being lucky in the order he killed people intact, to serve as a counterpoint to Little Bill, who made such a fuss about remaining calm?
It would be fitting if the story was warped beyond recognition. Munny himself has tried to desperately to forget all the stuff he did in his wild days. At least, all the stuff he recollects, which isn't much (or so he said). The stories are either lost to him, or some nightmare where he sees himself doing things without understanding why. Meanwhile, his friends remember, and spread the stories to their descendants, and all of them see these as great and impressive acts. it's the inverse of English Bob, where he presents himself as this noble gunman defending a dishonored woman, but in reality he was just some drunk, murderous jerk feeling proprietary over a woman. It would be in keeping with the rest if Beauchamp turns Munny into some almost heroic outlaw figure, while Munny would just as soon forget it happened, because all it reminds him of is how his best friend died for something he did.