rank means a lot, as we can evince from the text, and that rank is one of the main reasons for her disapprobabtion of Mrs Clay as a companion for Elizabeth (another reason is that Elizabeth slights her worthier sister Anne for Mrs Clay, as I have written in my previous post).

I don't mean to say that Lady Russell is a "snob" in the modern sense of the word (I don't know what connotation it might have had in Jane Austen's time), I was just replying to Delories' post, but that she is very attentive to social distinctions and this influences her opinion of Mrs Clay, as I think is evinced from the passages I have already quoted.
After all she is very good friends with Sir Walter, one of the most stupid creatures that ever was, but she overlooks his flaws because he is baronet, so people's characters only seem to mean so far to her: "Herself the widow of only a knight, she gave the dignity of a baronet all its due; and Sir Walter, independent of his claims as an old acquaintance, an attentive neighbour, an obliging landlord, the husband of her very dear friend, the father of Anne and her sisters, was, as being Sir Walter, in her apprehension entitled to a great deal of compassion and consideration under his present difficulties".

About the Musgroves: they might not have the background of the Elliots but they are just a step behind them: their landed property and general importance are second in that country only to Sir Walter's himself, therefore their situation is not comparable to a mere Mr Shepherd. It is like saying that if Anne couldn't get the very best, second best would do.
And anyway, if Lady Russell thinks Charles Musgrove worthy of the 22-year-old Anne, for the 19-year-old Anne she "might have asked yet for something more". What could have been "more" that the eldest son of the second most important man in the land? A knight's son? A baronet's son? I don't think she means "something more" in character, for Charles M. is said to be "of good character and appearance".