I think a good place to start is trying to figure out what happened in the unchanged situations in 1840/41 with no Barnabas, Julia or Eliot (and no Angelique). Here's how I see it.
The Judah Zachary-possessed Gerard Styles goes wild in his (Judah's) first attempt at revenge and ends up killing Tad and Carrie (and maybe a few more family members). Gerard is killed and Daphne, strongly under his influence, also dies from whatever means or reason. Maybe she commits suicide. The rest of the family picks up the pieces and plugs along. Maybe even Gabriel is dead, but Edith is still alive and she becomes mistress of Collinwood. One of her children (who were off at some boarding school when all of this happened) becomes the parent of Judith, Edward, Quentin and Carl. As for Quentin I, his son is dead; his wife is dead (maybe Joanna's ghost still killed her, maybe Gerard did). But his life is basically over. Maybe he hangs around, helping to pick of the pieces but that's pretty much for him. Maybe he becomes a recluse. Maybe he leaves to start life over again elsewhere. And maybe (so many "maybes") he takes his staircase through time as a means of escape from all that happened and ended up who-knows-where. Wouldn't that've been a great sub-plot?
But Barnabas, Julia, Eliot and Angelique changed all that. Tad is still alive; Gabriel and Edith are dead and their progeny most likely lost their place as primary heirs. It now goes through Tad's lineage. That's not to say there weren't Judith, Edward, Quentin II and Carl. For that matter, it doesn't mean there wasn't Jamison and his children Elizabeth and Roger and their progeny. They could've all been born, but not at Collinwood. They are now the distant relations living elsewhere. But the 1971 masters and mistresses of Collinwood would be an entirely different family. Everything that initially happened from 1841 to 1971 would've been wiped out. No werewolves, no spectres of Quentin II and Beth haunting Collinwood. None of it happened. Like I said, in my version of what-would've-happened-if-DS-continued, that's what Barnabas, Julia and Eliot found when they returned. The family they loved is gone (well, gone from Collinwood, but not completely gone). Those inhabiting the estate are all strangers.
Gerard