A caveat for those who haven't seen the show from 1897 on - this posting is spoiler laden.
[spoiler]But didn't they show Quentin and Daphne going away together at the end of 1840? Who's to say that they ever returned? And Tad might have died young, so that Collinwood passed to Gabriel's kids. Or, perhaps Gabriel was the older of the two brothers, so his sons naturally inherited.
I too have often wondered about these time paradoxes. Of course the reality is that the writers just didn't figure anyone would ever know the difference, as others have pointed out.
But in thinking about it all now, I prefer to think that all that time travelling messed things up, creating all kinds of parallel times. The Collinwood that Barnabas, Julia & Stokes returned to after the 1840 storyline HAD to be a parallel of the original, since Liz obviously had no knowledge of the problems they'd gone back in time to fix (Gerard, etc.).
You have to wonder what the meddling around in the time continuum (sp?) did to the storylines that had come before. For example, did the 1840 shenanigans negate the Leviathan story somehow? Angelique was killed in 1840. How then does Sky Rumson come into the picture in 1969? And what does this do to the Dream Curse subplot? For that matter, what is Stokes even doing returning from 1840 at all? Or at the very least why does Liz know who he is? Stokes, if I remember correctly, was first brought onto the show as Cassandra/Angelique's teacher. If she had died in 1840, how the heck did he come to know the Collins family?
Which brings me to the myriad time paradoxes Angelique's existence poses. When B and co return to 1970 from 1840, they shouldn't mention Cassandra to Roger, he can't have married her if she was killed in 1840. And does this mean Sam Evans is alive upon their return to 1970? Cassandra killed him.
Remember that the 1840 Angelique knew nothing of any other times except 1692 and 1795. And I think that 1897 Angelique doesn't seem to know about 1968/69. The Angelique who shows up as Mrs. Sky Rumson during the Leviathan storyline seems to know about everything but 1840. And my namesake Cassandra didn't indicate she knew anything about 1840 or 1897. Maybe she just deliberately withheld this information from the other characters.
Now if anyone had been able to transcend all this temporal mess, it was Angelique. But she seems to be as in the dark about all of this as Elizabeth, Roger or for that matter the lovable but eternally clueless Victoria Winters.[/spoiler]
Okay, this is all starting to make my head hurt.