But how can this be, you ask, if the NT Roxanne actually turned out to be a youknowhat? Well, let me explain. You see, she didn't start off as a youknowhat because the post-PT story which begins in 1995 and is linked to 1840 hadn't happened yet, so Barnabas wouldn't've gone back to 1840 in which case the 1840 Roxanne wouldn't've been, well, you-know-what, so therefore until that point the 1840 Roxanne would've settled down with somebody nice and had kids who down the line would've eventually produced the NT Roxanne who wasn't a youknowhat, so, in this case
None of the 1970 Roxanne made sense. At least in your scenario you've got the non-you-know-what being a descendant. But, since in the 1970 we saw, she WAS a you-know-what, there's really no explanation for how she got that way. Since at that point Barnabas hadn't gone back, presumably it happened to her in a different way. Or, if we assume there's some sort of cycle to time, so that he really has gone back already to cause it, then she would have recognized him. I'm sure they never considered any of it when they introduced her, and then they went back to 1840 and subsequently none of it made sense.
It always did seem funny, though, that they came up with the name Roxanne so soon after mentioning it for Willie. Either someone really liked that name, or for some reason it just stuck in their minds and they forgot they had used it for a non-appearing character.