The show did not know at the conclusion of the 1840 storyline that the ending was to be in April.
So, ratings had fallen after the Levithan storyline, but, when the 1940 Parallel Time story was airing, the ratings were actually rising.
It is reported that Diana Millay had been asked to and had agreed to return to the show, but the cancellation came that there was no time for her to return.
Also, I understand that Alexandra Moltke had been asked to return to the show also. And Keith Prentiss had a contract, but I am not sure for what length of time.
(I have suspected that the role of Joanna Mills was written with the possibility of the return of Ms. Moltke and that she would be playing the role.)
The puzzlement to me in 1971 and still today was why there were so many performers hired during the 1840 storyline but they were slowly being dropped - not used in the 1840 storyline but still Keith Prentiss was hired and was offered a contract with the show.
During the final months of the show, added to the cast were Donna Wandrey, Kathy Cody, Kate Jackson, James Storm, Virginia Vestoff, Gene Lindsey, Tom Happer, Keith Prentiss, and Lee Beery.
In addition, Clarice Blackburn, Jerry Lacy, Terry Crawford, Elizabeth Eis, and Humbert Allen Astredo (who had not been on the show for a while) were brought back.
I know that Denise Nickerson left, so the hiring of Kathy Cody made since. The departure of Katheryn Leigh Scott necessitated the hiring of Virginia Vestoff. (I understand that the character of Samantha was written for Katheryn Leigh Scott.) Ms. Scott's departure also necessatated the hiring of Donna Wandrey and the changing of Lara Parker into a sympathetic figure. (She was the most popular Dark Shadows actress during the show's last year, according to the Daytime Television magazine's poll.)
When the show moved over to 1850 Parallel Time, Virginia Vestoff and Jerry Lacy were quickly killed off. Not used at all were Tom Happer, James Storm, Gene Lindsey, David Henesy (who had left the show, but we did not know that at the time), Michael Stroka, Donna Wandrey (who may have been dismissed from the show - I am not sure), and Lee Beery.
I never understood why there were so many characters in 1840 but not in the Parallel Time.