I do not think anything would justify me in wishing you to sacrifice all your happiness merely to oblige my brother, because he is my brother, and who perhaps after all, you know, might be just as happy without you, for people seldom know what they would be at, young men especially, they are so amazingly changeable and inconstant. What I say is, why should a brother's happiness be dearer to me than a friend's? You know I carry my notions of friendship pretty high. (Ch. 18)

I do accept your theory that Isabella is projecting and gives herself away continually. It is hilarious, really! (;D) Adding to your lovely analysis, Isabella’s emphasis on the idea John’s happiness (her brother) is no dearer to her than Catherine’s (her friend) suggests she would like Catherine to sport reciprocal sentiments on her behalf. I cannot believe Isabella has any real desire to maintain Catherine’s favor and compassion for the sake of their friendship. It must be that she sees Catherine as a way of getting closer to the captain just as she used her to pump information about family and gain favor with James. Inconstant to her brother, to her lover and to her friend—Isabella is fickle indeed.