] Mrs Dashwood's reticence is not due to any custom. We are in doubt, and Elinor is in doubt, but Mrs Dashwood is not in doubt: they are engaged.

I agree--it's not due to a custom of the times, but it is consistent with Mrs. Dashwood's earlier behaviour concerning Edward and Elinor.

In Ch. 3, we have:

"No sooner did [Mrs. Dashwood] perceive any symptom of love in his behaviour to Elinor, than she considered their serious attachment as certain, and looked forward to their marriage as rapidly approaching. "

And from Ch. 4:

"[Elinor] knew that what Marianne and her mother conjectured one moment, they believed the next -- that with them, to wish was to hope, and to hope was to expect."

For Mrs. Dashwood there is no need to confirm something she is already perfectly certain is the case. The notion of forcing it out of Marianne goes against all her romantic ideals.

And Elinor knows her mother and her sister are like this. Yet, when it came down to it, Marianne did hint around to find out what was really going on with her sister and Edward, dropping little comments like "When you tell me to love him as a brother..."

and we learn that

"Marianne was astonished to find how much the imagination of her mother and herself had outstripped the truth."

Elinor can recall Marianne's exclamation "And you really are not engaged to him!" I think Elinor could also be certain that the conversation was reported back to their mother. And Elinor very clearly laid out exactly how things were and where they stood with Edward.

Although she knows what her mother is like, I think it must be genuinely baffling to Elinor that there is no attempt to find out how things really are with Marianne and Willoughby.

]Marianne has not told her, so Marianne must have a reason for not telling her.

Marianne's behaviour is also very baffling and inconsistent here, for Marianne prides herself so much on openness and honesty and revealing all her feelings. It's no wonder that Elinor feels "surprise...that she, or any of their friends, should be left by tempers so frank to discover [the engagement, if it exists] by accident.