] I thought Mrs Dashwood was very harsh with Elinor.

I think Elinor is extremely reasonable here: if Marianne and Willoughby are not engaged, it seems to me that Mrs. Dashwood should be very aware of that situation, so as to be able to combat any gossip, and also so she will know how to regulate Marianne's behavior.

However, that's ignoring the fact that she has never done anything to regulate Marianne's behavior, and she's not likely to start now.

I felt so sorry for Elinor here, and quite upset with Mrs. Dashwood, whom I usually view as a kindly but not very realistic human being. However, here she completely attacks her one daughter just so she and the other (certainly, it seems, her favorite) can go on living in their own little dream bubble. Badly done, Mrs. Dashwood! Badly done indeed!