Actually, none of what I posted was my speculation alone (I didn't dream it). It's okay if you delete it... I don't mind. It's nice that you are careful about what goes up here; and, also nice to let the poster know something might be deleted.
You could see the scripts tighten up as the "end" of the TV show got nearer. As the last story arc ended, there was no introduction of any new story arc. Normally, bits of the coming story arc would be blended in as the new arc was prepared. The last season was a "stand alone" season with no real connection to the whole series. On the front end, this was an attempt to change the direction of the series... from supernatural sci-fi to gothic romance. On the back end, you could argue the attempt failed; because no hints of the next story arc were introduced.
I think the original idea for "Night of Dark Shadows" was to have Virginia Vestoff play Samantha. This was in the spring of 1971. The series was going to feature Diana Millay as Laura Collins. My guess is that Diana switched over to NODS and changed "Samantha" to "Laura" because Virginia couldn't get away from "1776" to Tarrytown as easily as she could to the regular studio.
As far as "how" Laura could return? Remember, they were in "Parallel Time." No version of the Laura character had appeared in "Parallel Time." That being said, I don't think they would have stayed in the 1841 PT setting.
Yes, Diana Millay is someone I asked. I was surprised, too, at first, that she said she was asked to re-join the TV series as Laura in 1971. But, if you look at the entire picture of what went on with casting during the winter and spring of 1971, it makes perfect sense. She could have held up the daytime show while the others filmed NODS.
:- The term "regular" on DS originally meant being under contract. However, as the show went on, more "regulars" went off contract. They were called and worked day-to-day. This was good for the artistic freedom of the writers and the actors who happened to have other work. It wasn't so great if you needed a steady paycheck.... Later, though, I think "regular" meant all the actors associated with the show UNTIL they started turning down roles -- mostly when they got better contracts on other shows!
Of course, I'm just speculating! Take it with a grain of salt!