I first presented some of these ideas in The World of Dark Shadows #74, in 1997. The same disclaimer applies!
I'm convinced the history in which Quentin, Tad, and the others died in 1840 was not the original history. Another cycle is necessary to explain the background of Roxanne, and to account for Grandma Edith's having been alive and known what she did in the 1897 sequence (which of course aired before the 1840 sequence).
Here's my theory: when Judah Zachery wasn't possessing a body, his severed Head only came to life in time periods in which his betrayer, Miranda (aka Angelique), existed on Earth. And Angelique had skipped over the 1840 era. She didn't exist in that time until Satan sent her back--with her memory of future eras erased--as punishment for her having helped Jeb Hawkes destroy Nicholas Blair in 1970.
In the original history, the only elements of the paranormal in 1840 were Leticia's psychic ability and Quentin's construction of the Stairway Through Time. Here's my guess at what may have happened.
Gabriel murdered Gerard and Samantha--and got away with it, because his supposed disability seemed to rule him out as a suspect. Leticia had psychic visions of him as the killer. But she thought that meant that the real killer was someone for whom she cared deeply--that because she couldn't face it, her subconscious was substituting the image of a person who couldn't possibly be guilty.
Edith, however, became suspicious. She spied on her husband and discovered he was able to walk. Gabriel caught her and knocked her unconscious. He could have gotten away with this murder too; but he couldn't resist the idea of killing her in the same way his father Daniel had killed his wife, by throwing her off Widow's Hill. He dragged her out there, unconscious but still alive--and ran into a brooding Daniel! The two men struggled, and both fell to their deaths. After the tragedy, Quentin let Edith and her children stay on at Collinwood.
Daphne became well enough acquainted with Quentin to forgive him for his part in Joanna's death, but they never fell in love. She and Desmond married and had a happy life together.
Leticia was secretly in love with Desmond and grieved over losing him. But Flora's novel inspired by her life was a huge success, and both women became celebrities. Leticia went back to England and became a bigger music-hall star. Her fame was still remembered in the Nineties, when Pansy Faye deliberately copied her. ("Faye" was a stage name for Pansy, possibly for Leticia as well.) Later in that decade, Pansy's benign possession of Charity Trask would help bring out the submerged personality of Leticia--who was, in truth, reincarnated in Charity.
Tad Collins was killed in the Civil War, and his grieving father Quentin only survived him by a year or so. In that last year of his life Quentin adopted Edith's children by Gabriel--who would have been his heirs in any case--and to please them, married Edith.
So in 1897, Edith was the widow of both Gabriel and Quentin--and referred to both of them, in different contexts, as her husband and/or the grandfather of Edward, Judith, Carl, and the second Quentin. The husband who had died 30-odd years before, and had known the famous "secret" (the presence of a chained vampire in the family mausoleum), was Quentin.
How did Edith--or, for that matter, Quentin--learn a secret Daniel Collins had never known? Simple. In a world without Judah Zachery, Ben Stokes lived several years longer, and confided the secret to Quentin after he became master of Collinwood. Quentin shared it with Tad, and after his death with Edith or, more likely, her eldest son. Edith, aware by then there was a secret, pried it out of her son.
Roxanne was the sister of Quentin's wife Samantha. What became of her in the original history? That's where the Stairway Through Time comes in. Roxanne found Quentin's Stairway while she was at Collinwood--possibly fleeing someone, possibly just exploring. She walked up it--and emerged in the 1960s! Willingly or not, she stayed there.
This was the Roxanne who became engaged to Willie Loomis. It's a cop-out to dismiss that name as coincidence if there's any way to justify its being meaningful. Remember Willie's saying "his" Roxanne had a sister with the unlikely name Sue Agatha? I suggest he'd misheard her mumbling of "Samantha."
The First Revision of 1840 history came about when Angelique's presence in that era caused the Head of Judah Zachery to come malevolently alive--and hitch a ride to Collinsport with Desmond. "Gerard's" reign of terror led to the deaths of Quentin, Tad, Carrie, Daphne, and "Gerard" himself. Whoever was left alive failed to recognize the danger of allowing "Gerard" to survive as a ghost. He and the other spirits he controlled probably used the Stairway to travel directly to 1970 (ironically, the year Angelique had come from)--when "astral twins" of Tad and Carrie were living at Collinwood. That opened the possibility of restoring "Gerard" and Daphne to life, but the attempt to restore "Gerard" failed.
That much is fairly straightforward. The real puzzle is Roxanne.
After Angelique disappears from the 1970 storyline, Roxanne appears as a vampire. We learn she's been one since 1840. Daphne (in 1970) recalls vampire attacks in that year, and also recalls Roxanne as a young woman who "changed" somehow during the time she knew her.
We know how Roxanne becomes a vampire in the final history. Julia releases Barnabas from his coffin while he's still the "1840" Barnabas, he attacks Roxanne, and Angelique thwarts attempts to save her. But what happened in the First Revision, before Julia or Barnabas traveled back in time?
Here's my answer. In that version of 1840, someone released Barnabas from his coffin. Barnabas made Roxanne a vampire--at an earlier date than the one on which she had, in the original history, traveled to the future via the Stairway. So her time travel never took place; but now, she was capable of surviving until 1970.
When history was changed, Willie's memory of his ex-fiancee Roxanne was wiped out. That's why he showed no sign of recognizing her as a vampire in 1970. Apparently, whether characters remember the previous history after a change depends on what was changed, how involved they were, and how much of their memory would have to be altered. Willie and Roxanne had already broken up and he hadn't expected to see her again, so his memory of her could easily be excised.
Back in 1840, Barnabas was chained in his coffin again; but before that, someone erased his memory of the experience. If they hadn't, the 1970 Barnabas would have found himself suddenly remembering it.
I suggest that when Ben Stokes first learned Desmond had brought the Head to Collinsport, he released Barnabas to seek his help. An obvious objection: in the final history, when Barnabas has already been released, Ben tries to handle the matter himself instead of going to him. But in that history, Ben feels ill at ease with the time-traveling Barnabas. He no longer feels he really knows this man. Also, Barnabas and Julia are wrestling with a mysterious problem involving Gerard, and Ben doesn't want to burden them with another one. He has no way of knowing the problems are connected.
So in the First Revision, Ben released the "1840" Barnabas. And Barnabas--quite deliberately--made Roxanne a vampire before Angelique arrived in Collinsport. He helped Roxanne adjust to her "new life," and no one realized she had died.
Angelique, driven by jealousy of Roxanne, erased Barnabas's memory and chained him in the coffin again. To make sure he'd stay there, she killed the bothersome Ben Stokes. And then she went blithely on her way with Laszlo...unaware she had sabotaged her best chance of destroying her archenemy, Judah Zachery.
That's my interpretation of the 1840 backstory. To learn what I think was--and wasn't--accomplished by intrepid time travelers Julia, Barnabas, and Elliot Stokes, I'd suggest once again that you check out my fan fiction!