As CarolTS said walking alone with a man was fine but doing it with the same man often was not—then a preference was suspected. It is the same with carriage rides. In NA Catherine Morland rides out with John Thorpe twice but Mr. Allen points out to her that the second time was too much. Catherine also spends (a delightful) part of the trip to NA riding alone with Henry Tilney and there is no impropriety indicated by the narrator. All the other intimacies you point out were no-no’s and each instance of them goes to convince Elinor that Marianne and Willoughby are engaged. Note that Willoughby did not write to Marianne until she effectively ended their relationship by requesting her letters and lock of hair because had he done so it might have been construed as a sign he also felt they were engaged. I agree Mrs. Dashwood was remiss in her responsibilities towards Marianne. Elinor does her best in her place but it is little effective. Mrs. Dashwood might as well have been in another country as Brandon was to Eliza for all the freedom and secrecy with which she let Marianne and Willoughby run their affairs. Mrs. Dashwood like Marianne relied on the veracity of Willoughby’s behavior to explain his intentions. (:D)