All interesting posts... The sudden departure triggered a real disturbance among the Dashwood ladies.

As I see it, both Mrs Dashwood and Elinor are devestated by Willoughby's sudden departure. Mrs Dashwood is nearly overcome and Elinor suspicious and greatly disturbed by Willoughby's change of behaviour, nor can she understand his sudden departure giving no real reason.

Elinor appears to be an acute observor, evidence matters.
She feels the circumstance of publicly expressed affection of Marieanne and Willoughby is not a confirmation. To be an engagement, it has to be publicly acknowledged. Regardless of Mrs Smith, Willoughby and Marieanne need not conceal any engagement from them.
Yet Mrs Dashwood does not want these doubts. She easily explains Willoughby's departure in terms of Mrs Smith's disapproval of his connection to Marieanne and suppostion of an engagement is enough. Yet she is aware Elinor will have difficuties with her assurance and finds Elinor's feelings 'incomprehensible'.
For Mrs Dashwood, 'feelings' about Wiloughby's charecter are crucial to her view. However, for Elinor it is not a matter of feeling but of proof. She says she wants no proof of their affection 'but of their engagement I do.'