Now that is very interesting, MB. I had forgotten that you had posted something like this years ago when we were discussing the subject (sorry if I haven't taken time to find that earlier thread) ... though I didn't recall it as being quite so definitive. Though I don't doubt what you say, it does still raise some questions in my mind. If events did transpire as Don Costello apparently said, then why didn't Dan Curtis, in the apparently recent interview I quoted above (which I haven't seen but was transcribed from one of the DVD releases), affirm this? I do remember that Mark Rainey has said that DC told him and his co-writer to make Elizabeth as Vicki's mother in their novel "Dreams of the Dark." DC's directive to them wouldn't necessarily mean that was always the case with the TV show, though.
The other questions I have are: if Art Wallace did change his mind on this regard, did he ever confirm this in an interview - or was he ever even asked about Vicki's parentage? And, finally, did Francis Swann, who is credited with numerous scripts from this period, ever confirm this in an interview? Or another writer, for that matter? I think this last and third question probably can't be answered as I believe Swann left the show and was not interviewed in DS fandom.
I'm not trying to play the devil's advocate, but I just wonder if there is anything in writing anywhere, where any of these people are concerned. A rather curious (to me) thought is whether Robert Costello's remarks are so widely known that this is the reason why the "Elizabeth is Vicki's mother" is the prevaiiling view. I had never heard this anywhere except in the previous online discussion (which I had completely forgotten). And so far in the present discussion, no one else had pointed to Robert Costello as the source or evidence for this point of view.
Again, I'm not trying to argue, because this does seem to make a convincing case - moreso than the evidence from the show itself, though, IMO. And of course some of the evidence on the show itself is open to interpretation, or else there wouldn't be this discussion. I'm willing to accept outside evidence if it does truly depict the intentions and agreement of the writers, producers, and directors. And unfortunately, we know that they haven't seen eye-to-eye on all occasions (e.g., Art Wallace & Dan Curtis). I really don't know anything about Don Costello, and I'm not trying to impugn what he said, just hoping for some confirmation since some questionable things have been related orally (and in print) by various DS alumni ...