This is the problem with favorite books. I went searching for that last quotation (the "overmastering" one) and found this:
". . . Detachment is a rare virtue, and very few people find it lovable, either in themselves or in others. If you ever find a person who likes you in spite of it - still more, because of it - that liking has very great value, because it is perfectly sincere, and because, with that person, you will never need be anything but sincere yourself."
"That is probably very true," said Harriet, "but what makes you say it?"
"Not any desire to offend you, believe me. But I imagine you come across a number of people who are disconcerted by the difference between what you do feel and what they fancy you ought to feel. It is fatal to pay the smallest attention to them."
"Yes," said Harriet, "but I am one of them. I disconcert myself very much. I never know what I do feel."
"I don't think that matters, provided one doesn't try to persuade one's self into appropriate feelings."
It'd be unfair to Sayers' genius to say that Miss de Vine comes across as anything less than a fully-realized character in the book. However, I think she really does play the part of Shakespeare's Fool or a the Greek Chorus, telling you what you are to make of the story - or at least where you ought to start. Almost all the magnificent pronouncements of the story seem to come from her.
Peace of Christ to you,
ETA: fwiw, I find myself myself much closer to Harriet's "I disconcert myself very much" than to the divine peace of Miss, um, de Vine.