I cannot see quite how Mary would not know if Louisa is prepared to mention it so casually to someone outside the family :-) It is not as though Louisa's mentioning it is such a clever move on her part in attracting in Wentworth that she makes a tactical decision to tell what is in fact a secret within the family - she just casually drops it into the conversation to distance herself and her family from the 'Elliot pride'.

It is possible that in her weaker moments, alone with Charles, Mary berates him with it. But even she perhaps has the sense to see that it would not reflect well on her to mention it in public. I think everyone just regards it as a practical matter - the Musgroves are an old country family of respectability and large fortune and in the first class of society in the country [ie Somerset] and so Charles was a suitable candidate for either of the youger Elliot daughters. It was just a matter of sorting out which and I am sure he would have discussed every step of it with his parents. No broken hearts involved!

In terms of who knows what in the Elliot household, I am not sure Anne was ever party to discussion about Elizabeth and Mr Elliot either - I am sure she would have surmised Sir Walter and Elizabeth's plans, but would never have been included in any discussion about them before or after their disappointment.

You are right - thinking about all this points up another contrast between the Elliots, with their secrets, unsuccessful aspirations (think of Sir Walter's unreasonable applications as well as Elizabeth's hopes) and heartaches, and the Musgroves, where all is open and talked about and such matters are discussed in a fairly matter of fact way.

Imagine going from one household to the other at 19, as Mary did, with no mother of your own to turn to for advice and help in adjusting your ways to such a different setting.