....even if she has just refused him.

It's certainly true that he's courteous in his manner of addressing Wickham. And Miss Bingley certainly isn't. I think she is definitely being over-familair when she calls Lizzy 'Miss Eliza'.

I agree with you in that Darcy really means it when he gives Elizabeth his 'best wishes for her health and happiness'.

A while back on the P&P board there was a discussion about this remark of his and Mr. Collins's remarks to his cousins when leaving Longbourne after his first visit. He wishes 'them health and happiness, not excepting my cousin Elizabeth.' (Chapter 22.)
When Mr. Collins says this, I think that he is doing so to show that he has no hard feelings towards Elizabeth - in order that the Bennets may be struck by his good nature and forgiveness. In other words, he just wants to show off!