Mrs. Bennet's rebuke to Elizabeth regarding her "running in the wold manner she is suffered to do at home" seems to me an indication that Elizabeth is being too flirtatious with her sister's beau. Now we all know that Lizzie has no intention in flirting with Bingley. She is merely being her witty, vivacious, delightful self. (Yes, I am a little bit in love with Elizabeth Bennet.) Mrs. B who truly is one of the dimmest bulbs on the face of the earth cannot see a woman conversing with a man except in the former hoping for romantic notice from the latter.

Darcy then observes that there are fewer opportunities for Lizzie to study deep and intricate characters, indeed, to find much to study since there are fewer people (presumably of her class) in the country.

Elizabeth replies that, over time, people's personalities and behaviour are subject to change and these alterations provide new sources of interest to her.

Now we have Mrs. Bennet's reference to "that" being as ubiquitous in the town as it is in the country. It is my belief that Jane Austen wishes to convey that Mrs. Bennet is totally unable to follow sensible conversation. Like her sister Phillips, Mrs. B infers something vulgar is being talked of.

Mrs. B goes on to champion the country as if it had been insulted, reaching out to Bingley for support. Bingley, in his turn, shows how much he resembles Jane in his reply. Bingley tries to satisfy all points of view by stating that town and country are both highly satisfactory to his point of view. Mrs. Bennet rudely attributes denigration of the country to Mr. Darcy.

Embarrassed, Elizabeth tries to explain what Mr. Darcy actually said. She does not "stick up for him" so much as she seeks to clarify the topic under discussion.

The topic of poetry is raised by Mrs. Bennet as further praise of her eldest (and hopefully soon-to-be-married) daughter. I believe Elizabeth is embarrassed again by her mother's lack of subtlety, sense, and decorum. Elizabeth tries her best to lighten the mood by speaking playfully on the topic of poetry as one of the "foods of love."

Mrs. Bennet proceeds to open her mouth again and again only to change which foot she will put in it next until she, Kitty, and Lydia, take their leave.

In P&P1, Priscilla Morgan's performance in this scene actually makes me squirm with embarrassment. Ms. Morgan is gifted but the true genius is Ms. Austen.

I hope this helps.