Your art/nature antithesis can be applied not only to characters. One very well known example is the idea behind landscape gardening. From what I can make out the important thing is to synthesise the two - as you pointed out in your post.
When Lizzy and the Gardiners see Pemberley for the first time, they are impressed because the combination of art and nature is so perfect.
"in front a stream of some natural importance was swelled into greater, but without any artificial appearance. [...] Elizabeth was delighted. She had never seen a place for which nature had done more, or where natural beauty had been so little counteracted by an awkward taste" (ch.43)
I think the impression is supposed to be one of unadulterated beautiful nature. But there goes a lot of work into creating this impression. I think of it as 'groomed nature'.
I'm looking forward to your analysis of Lizzy and Darcy.