When I think about the quotes you have listed, I think that in a sense, perhaps Mrs Dashwood was not out of line in jumping to the conclusions that she did.

Why shouldn't she expect her stepson to honor his promise to his father?

As far as Edward's attachment to Elinor, once she was roused from her mourning to pay attention, she discerned his fondness for Elinor just as Fanny did. It seems rather natural that she might expect a forthcoming marriage. She may not have been privy to the intimacy Elinor and Edward had, which seemed to vary back and forth to where for some painful minutes, she (Elinor) believed it to be no more than friendship. Though to be completely rational about it, Mrs. Dashwood might have foreseen some problems with Fanny's family.

I guess what I'm saying here is that sometimes your "feelings" or instincts direct you right (and sometimes wrong too). I don't really see anything wrong though in Elinor requiring proof though.

In this reading I was really struck by the insensitivity of Marianne in the Elinor-Marianne conversation in Ch. 4. Elinor really was quite frank, and laid out her feelings and fears. Just after Elinor lays out her fears "unsure of his regard...many difficulties in his way, if her were to wish to marry a woman who had not either a great fortune or high rank." , Marianne comes back with "And you really are not engaged to him!". I'm thinking, didn't you listen? Marianne was presented with reason to doubt, and still jumped to conclusions.