] ] Although it does seem a bit intimate, like the way they call him "Willoughby" even though he's not related to them or officially engaged to Marianne. Do you think perhaps Marianne and Mrs. Dashwood are rushing the relationship a bit?

] I had always thought it ok becasue Willoughby was his last name.

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.