Hi Para,
generally, any effort to make the "facts" of the Dark Shadows narrative fit an internally consistent structure is doomed to failure. There was really no effort made to establish continuity as the story evolved (or unraveled!), mainly because Dan Curtis was the de facto "head writer" on the show and would change the direction of the plot on a whim (and, if the screenplay writers protested, he'd ask them "Well, are you writing the show or aren't you? I can get other people to write this show if you're not interested!). There was also the problem that they were writing it all very much on the fly and they usually didn't have time to go back and check through the voluminous scripts of bygone years (though Sam Hall did keep a set at home).
To answer your questions:
[spoiler]1. Rose Cottage was identified as "the old Magruder property" in 1970. I see no reason why it would not still exist in 1971 since Gerard's ghost never haunted the grounds in '70 because of the change of history, so he would never have used magic to burn it as we saw in the Summer of 1970 storyline. However, bear in mind that sometimes even when the characters went back to change history, some of the events that SHOULD have been changed still happened. Thus, when they came back to 1969, Liz and Amy and others still had memories of Quentin's ghost haunting the grounds, even though he never died so the haunting should never have happened.
2. The Stairway in Time was created by the 1840s Quentin Collins (not the same as the 1897/1960s Quentin. Check out an online document called The Dark Shadows FAQ for further lore about it. It basically follows the whim of the writers!
3. Gerard dies at the end of 1840.
4. Professor Stokes finds out that Collinwood was saved as a result of his, Barnabas, and Julia's actions in 1840, when he returns to 1971 with them.
Overall, the 1840 storyline has more conflicts with pre-existing continuity than any other story--mainly because it came at the end of the show so there was more story around for it to contradict. I'd say you should feel free to write any story you choose to devise about that plotline. Nobody could possibly make more of a mess of it than the writers already managed to do on their own! [/spoiler]
Best wishes,
Gothick
