I don't know about the destiny thing. If you had to invent such a thing to make it all work, if you were writing more DS after 1970, then you might have to resort to such a vague idea, and you could get away with it on a show about magic.
[spoiler]The weird thing for me is that '70 Roxanne shows no recognition of BC, and vice-versa, not even a psychic feeling from running into an old victim. It seems clear to me that R in 1970 was meant to be a victim of BCs, unless they were just winging it, and the writers really had no idea yet why R was a vampire. Well, plenty of people can hide their feelings, and Roxanne would be no exception I guess. Usually, though, we get to see an occasional private moment where the character's mask slips, and we don't get that with her.
In general... everything in 1970/71 is changed by BC's altered 1840, so much that they didn't dare even attempt to show real "present day" events after that. No Roxanne since she died in 1840. No Cassandra. No Barnabas. You see the perils of pursuing this line of thought too far.
Here's the real Roxanne problem: 1970 vampire Roxanne only existed because of BC's going back and interfering with history. All the other 'present day' events that sprung from the past (on DS overall), before any BC time-travel, were events that came from a non-interfered with, "pre-Barnabas" timeline. BC in the original timeline was chained in the coffin in 1840. It's all well and good to speculate that there was another vampire around (and maybe you have to assume it to make things work, but it's messy and I don't like it) but we see no evidence for it. Though I suppose vampires could be like hoboes, staying out of each others' territories... maybe BC psychically deflected the competition away, just by being there.
Anyway, we see the results of BC going back in time... Roxanne is made undead (so far, so good) and then destroyed... both results of BC going back, but only the first event existed in the original 1970 timeline we saw. I have to guess at three possible timelines here. One, BC is chained and no Roxanne reaches 1970. Two, the original 1970 we saw. In this timeline, BC isn't alerted to the death of "Julia Hoffman Collins" in time, doesn't I Ching back to stop himself strangling her or biting her... Roxanne stays out and about after death and survives to a ripe old age. Angelique also doesn't die, and goes on her way after checking in at the crypt. Third, the timeline as changed by 1970 Barnabas.
I'm going to have to come back here later, and see what gaping holes I may have left in this thing. My brain's done wore out.
Anyway, what I was getting at with the three timelines is that I think there may have been an original timeline we never saw. Somehow, BC and JH chose to change that history, in a way that produced the second timeline, the one we think of as uninterfered with, the "original". They have no memory of it, because it of course has now never happened. The second timeline is also screwy however, and they interfere again, not realizing they tried before, which of course they didn't, because that never happened... anymore.
You know, it just hit me that you might have to have four timelines to make this work. Suddenly I feel like Einstein, making up all sorts of extra dimensions.[/spoiler]