That sounds like a reasonable answer as to why 1840 doesn't exactly mesh with 1897.
This has just led me to think that 1897 Edith may have actually been married to Quentin I, who died in 1863, and she was Tad's mother (The Thaddeus Collins who fought in the Civil War) in addition to the son who had Edward, Judith, Quentin II, & Carl.
Anyway, really, I'm starting to think that the Stairway was responsible for the "temporal bubble" that this sort-of alternate 1995-1970-1840 existed in, and that once Desmond chopped it up, the bubble popped, and things went back to the way they'd been "before". So, B&J's actions in the "past" didn't save Liz and Collinwood, but rather Desmond's chopping.