OK, since we're doing spoilers here, I'm going to refer to [spoiler]Jenny's murder of Quentin[/spoiler] as Event A, and I shall refer to [spoiler]Quentin's haunting of Collinwood[/spoiler] as Event B.
If I remember correctly, Event A was not in any way caused by Barnabas's intervention in 1897. Therefore, in order for Event B to happen, in the original course of events in 1897, Event A must have been reversed without Barnabas's intervention, since Event B was caused not by Event A but rather by events that happened later in 1897 (Events C, D, E, and so on through the alphabet and beyond). But all Barnabas knew at that point was that Event A happened, and that Event B happened, and that it was logical to suppose (Barnabas and logic...hmm...) that Event A caused Event B and that once Barnabas had undone Event A, everything was fixed, and he could wash his hands of 1897 and concentrate on getting back to 1969. Barnabas did not make this assumption, however. (Barnabas and logic...grr..)
In the original course of events in 1897, how did Event A get undone?