I agree that Elizabeth skates pretty close to the edge sometimes, but we are told in ch.10 that a) "there was a mixture of sweetness and archness (IMO we would say "teasing") in her manner which made it difficult for her to affront anybody" and b) "Darcy had never been so bewitched by any woman as he was by her".

So, Elizabeth has a *non-verbal* way about her that prevents her from hurting people's feelings, plus Darcy is already so entranced that she can do practically no wrong in his eyes!

Now that you mention it, I also wonder what other evidence there is that she "willfully misunderstands" people!