Most sources I've found state you should add the possessive apostrophe even to nouns ending in s, as in Thomas's and James's, but does this ring true for nouns such as duchess, countess, and marquess? Can one write countess', duchess', or marquess' instead?
Cosmopolitan dropped the possessive apostrophe here. Now, they've also spelled duchess with a capital D, and I'm not sure if that may have something to do with it. They aren't a reliable source anyway, but I'm just wondering if the lack of the possessive apostrophe may be permitted for nouns such as those I've enumerated above?
And would it be acceptable as part of a title (any title) and/or royal title/rank?
I'm mostly interested in the British English perspective.
 I found this article from the Telegraph and they've used duchess's. Same for this link. Shakespeare says Auvergne. The COUNTESS's castle. in The First part of King Henry the Sixth. Sally Gardner's book is titled The Countess's Calamity as well.