I do not think Mrs. Dashwood knows about the Allenham visit. I would find it hard to imagine Elinor or Marianne mentioning it, and their mother's reaction not being recorded.

Marianne's recovery of some feeling for Mrs. Smith's rights, and, perhaps, of how it looked to be alone with Willoughby for so long, give me more hope for her future amendment than we can collect so far. I wonder if Willoughby and she would agree that it might not have been prudent after the fact?