Brava, Mystery Author! (Though actually, I have a pretty good inkling who you are...). This is a novel and highly-entertaining story that weaves through canon and provides such breadth and depth to minor characters; it is very much the essence of Beholder :-)
I adore your 'Voluptous Lady's' narrative voice - her choice of language; her large and rambling parentheses that always add something lovely to the story overall; and perhaps most of all, her relationship with, and reflections on, Violet, which are a perfect portrait (excuse the pun) of inter-class sweetnesses and tribulations. The disjunct between them is done subtly, but it's always there: Violet will fetch the wine, Arethusa will pour; Violet won't think to leave the comfiest cushion for her friend, Arethusa will be too gracious to mention it. Over and above that, though, comes the great fondness that they both must feel for one another, and balancing the two is a splendid feat. As others have mentioned, I think my two favourite lines (and their juxtaposition) are:
People of Violet's class always respond well to directives; they aren't at their best when the initiative must be theirs. This is not a snobbish statement, but merely one of fact.
and
A little shiver goes through me. It isn't often that Violet becomes masterful, but I find it rather exciting when she does. !!!
I also adore the backstory you have given to the other portraits and the world-between-the-portraits, as it were, and both the Hooch/Sinistra story and focus on Minerva's grief are heartfelt and fulfilling. Overall, a marvellous piece!