Yes, this "breezy attitude" is annoying and disrespectfull to you and your business and then to think youare stupid enough to believe her "oh didn't we tell you last week?" Really?
They are leaving. They know the closing date as they have sold their home and I will assume have already know where they will be living. They are just keeping you around until they definitely know they wouldn't need you.
As in my contract, I want 2 weeks notice. However, a little courtesy would be nice to give you more notice since this move is going to happen and they know when so you can agree to an end date and fill that spot.