] Think of Emma's horror when Mrs. Elton, upon only having met Mr. Knightley once, refers to him as "Knightley." That degree of familiarity usually only occurs after a fairly intimate acquaintance. Here, I think it's JA's way of making a definite point.

I agree, and I think the point is that the Dashwood ladies had established an intimate acquaintance with Willoughby.

"He came to them every day."

"Nothing could be more expressive of attachment to them all, than Willoughby's behaviour.To Marianne it had all the distinguishing tenderness which a lover's heart could give, and to the rest of the family it was the affectionate attention of a son and a brother. The cottage seemed to be considered and loved by him as his home; many more of his hours were spent there than at Allenham."

There is genuine affection being displayed here, not just to Marianne, but to the entire family. And this is what makes his departure under such suspicious circumstances so painful. Even Elinor says "I love Willoughby, sincerely love him; and suspicion of his integrity cannot be more painful to yourself than to me."