The return to the present time 1971 reminds me of the same problem the end of Back to the Future had.
At the end of "Back to the Future", [spoiler]Marty McFly returns to a revised timeline where his family had a happy/positive life as opposed to the negative one we saw at the beginning of the film. This is after he goes to 1955 and changes history ( inadvertently) for the better. In the new timeline, there is a revised Marty who time travels to 1955, from the postive 1985. We even see this as his "present" self arrives in the present time a few minutes before his "past" self goes back in time.
When Marty wakes up at the end, he seems to be surprised when he sees that things have changed when no surprise should have been on his face. He shouldn't remember the original timeline as that one no longer exists, so there shouldn't be any reaction at all other then he is back home safe and sound. This is apparently for the sake of the audience to confirm that severe changes have been made and to see it thru Marty's eyes.
Now, there has been a revised Marty who goes back to 1955, and apparently a slightly revised third and final 1955 we do not see. This is the one flaw of the movie that always got me, but I suppose had to be done as movies need big endings to hit a note with the audience.[/spoiler]
Now, on to Dark Shadows and the return of Barnabas, Julia, & Professor Stokes to the present time from 1840.
[spoiler]In this revised timeline, there is a peaceful summer of 1970 without any hauntings of Gerard & Daphne and a Barnabas, Julia, & Stokes who have existed in this revised timeline. This is evidenced by Elizabeth's remark that the three of them know all about the night's Historical Society opening that has been months in preparing. She does not seem startled that they have suddenly appeared after being gone for five months. There has been a Barnabas, Julia & Stokes that Liz and the others have been seeing, probably since Parallel Time ended.
The trio we see here should no longer have memories of that violent haunting of Gerard but they do, again it is largely on the writers part going for full drama. They apparently did go to 1840 as they undid all of Gerards ghostly existence.
Most likely, they did go back in time as we see in 1840, but perhaps for a different reason. One of them most likely saw in an old diary that there was a Barnabas, Julia and later Eliott Stokes who took part in the events of 1840 and realized that they had to go back in time for some reason to make sure history went along as written. And the events played out as we saw, but maybe slightly altered. Like BTHF, there is a third and final 1840 sequence we do not see based on the three of them arriving from a peaceful present time and trying to sort things out.[/spoiler]