Actually I see John Dashwood as fairly steady - using the word to indicate that what he does stays consistent. He is interested in himself and his family - and only (basically) in those things. And so far within the first 8 chapters that's been played out again and again. I think JA is playing with us - she's playing with our understandings of the words. She uses the words at first as though they're going to be positive characteristics, but when you think about it, they don't have to be. Mrs. John Dashwood, on the other hand, she doesn't use any ambivalent words at all.