I never saw her as unsympathetic as others seem to have done. Ms. Austen specifically lists a number of positive qualities, and she is certainly a good friend to have moved near Lady Elliot and helped to raise her children.

Her chief negatives, noted in Chapter 2, are that she is a woman rather of sound than of quick abilities, and she has an exaggerated value for rank and consequence. Perhaps this was one underlying source of her disapprobation towards the match between Anne and Captain W. He doesn't have enough money, but he also doesn't have enough rank.

While speculating on Lady Russell's motives for interfering with the match, I started to imagine her relations with her friend, Lady Elliot. Certainly Lady R. would have become aware that Sir Elliot was not a good match for Lady E., and that Lady E. became dissatisfied with her husband. Perhaps Lady R. imagined that Anne's mother would have said something like...(I hope it's pardonable to quote from another Austen novel) "My child, let me not have the grief of seeing you unable to respect your partner in life...."

Your point of Lady R. and Sir E. not marrying because Lady Russell was aware of what a dolt he was is a good one, it hadn't occurred to me before.