Her "cold civility" reminds me further of the analysis in my Barnes and Noble edition of S&S. Part of the analysis discussed the characters' ability to use language in subtle and manipulative ways, to ensure they don't reveal too much. It seems like Lady Middleton has picked up on this ability (to a point)in being able to seem perfectly civil yet be perfectly poisonous underneath!
Although I must say, I never saw her constant reprimands of Sir John as being nasty; rather I thought JA was making a joke about nagging wives. I wonder how Sir John dealt with nagging? ;-)