Hehe, that comparison did occur to me, although I've never read any of Orczy's books (I was put off by the Royalist setting, although that can apparently be overcome if you're as good a writer as Broster :P)

Also: characters finding happiness later in their lives seems to be a theme in Broster's writing.

Yes, definitely—and the richness of her writing does such a good job of making that hard-won happiness feel earned, too. She gives a beautiful depth to it, especially in this book!