Coming late to the discussion, but I can add a few observations: Elizabeth is addressed as "madam" by both the butler at Longbourn, and Darcy. Elizabeth also addresses her mother as madam. That indicates to me that it was a universally used title of respect for a lady, the full form of "ma'am." I think it would be perfectly right to have servants address their mistress as "madam" when speaking to her (married or not), although "mistress" is also a possibility. Think of "madam" as the equivalent of "sir," and "mistress" as the equivalent of "master." I imagine that a servant who addressed Darcy might sometimes call him "master," but would probably use "sir" more often. I get the feeling that calling them by their names, Mrs. Bennet or Mr. Darcy, would be considered an impertinence from anyone but the housekeeper or the butler.

I also can't help but observe (related to another post in this discussion) that although "Miss Elizabeth," "Miss Mary," etc, were appropriate to use when in company with the sisters all together, as a way of distinguishing who you're talking to, when you had them alone, "Miss Bennet" was always the most proper. Darcy always called Elizabeth Miss Bennet when speaking to her, and even when speaking of her to someone else he said "Miss Elizabeth Bennet," rather than just Miss Elizabeth. Of course, Darcy is kind of a formal guy, but the only person I can find calling her just "Miss Elizabeth" in the whole book is Mr. Collins, who's her cousin, and also odious. Sir William calls her "Miss Eliza," but as he's older and has known her her whole life, he's entitled to a little more familiarity.

As far as who would be considered mistress of the house among several sisters, I really think it would be up to the master of the house to decide that. Presumably he would pick the sister he considered the most competent as well as the most worthy to hold that title. But if, for instance, Caroline had been keeping house for her brother for a while, and then Mr. Hurst died, and Mrs. Hurst decided to come back and live with them, I don't think she would be entitled to automatically take over management of the house just because she was older or widowed. She would be coming back into an already formed household, and although she was received as a member of the family and not a guest, it doesn't follow that she had any right to be in charge of anything unless the people who were already part of it decided to put her in charge.