]One thing that struck me in ch.21 was that Olivia's, not Lydia's, was the expected reaction at the time

I have been thinking about Lydia since about chapter 17, and I am glad you pointed out this aspect of it. Another item I found interesting was how seriously they appeared to take secrecy back then (at least in literature). How would plot lines ever advance unless secrets were kept. ;-)

One phrase from Olivia I found rather shocking (I keep comparing things to JA), was when she told her father of the two women living in "contented prostitution." Since JA described touchy situations without necessarily using the exact labels, I was surprised to hear Olivia say this directly to her father. But perhaps since he spoke often on his favorite topic of monogamy, they were already conversant on the subject.