] As far as I can tell, she's just trying to defend country living, without really understanding what the argument is.

I think Mrs. Bennet is trying to defend the country, but I also think she is also doing her best to disagree with anything Darcy says as she finds him such a proud and disagreeable man. What I think is ironic is that Lizzy tries to soften her words by giving another possible meaning to Darcy's words (in other words giving him the benefit of the doubt). She can see that it is not a good thing for her mother to disagree with him just for the sake of disagreeing, but she can't see that she herself also takes everything he says in the worst possible light.