Sorry folks, it was bad writing! The show was ending, a lot of them wanted out at that point, so they ended 1840 in a senseless manner & didn't care.
David.... when we talk this way, we're not talking about what the intent of the writers was. We
know that they just came up with a passable end and didn't think much beyond that. We're speaking
fictionally, assuming the DS universe to be real, and trying to make sense of it. To do this we have to go way beyond anything they had in mind at the time. Fandoms do this...
And I don't think 1840/41 had the worst writing or worst plotting... many other eras are very competitive in this area, including some fan-favorite storylines. Also, there are
bound to be contradictions in time travel stories. The very idea of time travel and history-changing makes the paradoxes come spilling out.... I think this was a very interesting way of dealing with the return to 1971. The worst, most thoughtless time stuff happened in 1968 probably, in particular [spoiler]Barnabas's coffin re-acquiring chains and transporting Barnabas back to the present from 1796.[/spoiler]