Here is Sotheby's reply in its entirety, with my comments below:
Thank you for sending those attachments. Having looked at the negatives we hold, they as you say, may be from the same sitting but it is impossible to tell if they are actually by Beaton. We have other examples of photographs very similar to Beaton’s but not actually by him. As far as we can currently tell, Sotheby’s do not own any copyright in the image you have acquired. However, we cannot give any assurances about copyright that others might hold in the image. Sotheby’s wouldn’t be interested in buying the negative from you.
With best wishes
Sotheby's Picture Library / Cecil Beaton Studio Archive
So, that's it. Not a lot of info in that message, and a little unclear in places. They seem to be saying that if they don't have an exact match in their collection, then they can't say for sure whether it is by Beaton or some other photographer who may have been there at the same time. Fair enough, from a legal perspective.
It appears (from Wikipedia) that Sotheby's acquired nearly everything of Beaton's a few years before his death. I will continue to pursue other avenues, if I find them, to determine whether anyone else might hold copyright over this image, but I would guess that the chances are slim. At least I now know that Sotheby's won't bother me about copyright issues.
I know, of course, that I will never own the copyright (either someone else owns it, or it's in the public domain), but until someone tells me otherwise, I will consider it my image to use however I like, and sell prints to anyone who wants them (while keeping the negative and large image files for myself). I will refer to it as a "presumed Cecil Beaton photograph, recently discovered."
If there's anything I'm overlooking here, please comment.
Btw, the 8x10 prints I had made for eBay sales, I had done in cyan blue, because it looks cool that way. It also looks good in sepia. But for historical accuracy, at a certain point I suppose I'll have them printed in b&w.