And until 1840, Sam & Gordon were very careful to respect events in the show's history as they affected different stories.
Uh, what show were you watching?
I understand that your point is that 1840 seems to have thrown more established plot out the window than other storylines had and those changes affected several previous storylines in a big way. I honestly do get that.
But to say that the writers were very careful to respect events in the show's history isn't quite the case. And one of my favorite examples is how Jamison/Petofi and Aristede managed to find the Collins Family History book in 1897, despite the fact that Eve/Danielle Roget travelled back to 1796, brought it forward to 1968, and gave it to Vicki as a wedding gift. Not only did Eve/Danielle remove the book at a point in history prior to 1897 - but it had also been removed at a point in DS' own storyline timeline prior to the 1897 storyline taking place, so we can't even argue that Eve/Danielle may not have travelled back to 1796 prior to what we saw in 1897.
The writers managed to screw up all sorts of little things like that because they simply didn't remember what had come before. I marvel at the fact that the writers themselves have even admitted that sometimes they'd go to ask the kids outside the studio what had happened in a particular instance because they couldn't remember the events themselves!