]in one scene at Longbourn where Kitty says "...and I don't see that Lydia has done anything so very dreadful"? Did Andrew Davies get this detail wrong?

I think so, b/c from what I learned in this GR and in the discussion of this matter in P&P board, I understand that elopment is a very objectionable in that era and I don't think a girl could say such a thing in front of her family even if it is her opinion of the matter,especially that I think at that scene Lydia and Wickam are not married yet!