And so I presume Mrs. Clay is "guileful as the serpent," where Lady Russell (arguably) might represent "reasoned conversion"?

Yet JA makes an argument here as well as in P&P for the influence of friendship. One proof of friendship and affection is the willingness to be persuaded against ones previous inclination (obviously depends on seriousness of the issue). The proof that Elizabeth does not care for or respect Lady Russell is the latter had "never succeeded in any point which she wanted to carry, against previous inclination." I don't think that is just LR's definition of affection, because we also see Lizzy making the argument in P&P.