Did Hardy make a mistake in The Mayor of Casterbridge, or is there one last plot "surprise"?

When I read early on in The Mayor of Casterbridge that the sailor, Newson, who bought Henchard's wife at auction was lost at sea I knew one thing for sure: he'd turn up again. And when he shows up again toward the end of the novel, visiting Casterbridge after a long sojourn abroad, looking for news of his abandoned wife and daughter, and Henchard expresses surprise, all I could think was: Man, have you never read a Victorian novel? So, yes, he turns up again, and through an almost comic series of absurdities tracks down the family he'd abandoned and Henchard, seriously jealous once again about this rival for daughter Elizabeth's affections, tells him that Elizabeth has died and is buried next to her month - Newson takes off without a word. How likely is that? In any event, Henchard, feeling guilty about this lie, follows but just misses Newson who departs on the next coach. So for the next two years (?) or so, Henchard lives in dread of Newson's return - figuring he may eventually learn that Elizabeth is alive and well. When this does finally come to pass, Henchard, terribly guilty, says good-bye to daughter and wanders off, walks several days to scene of the original wife-auction crime, and then takes on manual labor and keeps an ear open for news of doings at Casterbridge. OK, he's about as low as he can get - but as noted in previous posts, not through any tragic failing but through Hardy's believe in the grinding forces of fate and also through his own absolute stupidity, jealousy, insecurity, and duplicity. He's a pathetic figure but not a tragic one. This leaves, for me, with about 20 pp. to go, one last unresolved bit of plot: is Elizabeth in fact really Henchard's daughter? The only "evidence" to the contrary was the letter (not to be opened until her wedding day) that ex-wife Susan left for Henchard. But we learn, rather late in the story, that Susan was functionally illiterate - so she couldn't have written the letter. And who could have, or would have wanted to? I can't quite figure that out - and it may be a slip of Hardy's hand (Homer nods). In any event, I suspect Henchard will be welcomed, in some sorrowful manner (perhaps on his death bed?) back into the reunited family - Elizabeth, married to Farfrae as she should have been in the first place, and Newson her actual father or at least kindly stepfather (despite his abandoning his family for a dozen years for no apparent reason).