think of Lizzy and Mr. Darcy as being secretly engaged (she expected Lizzy to get a letter from Darcy, soon after their arrival at Longbourn from Pemberley), or did she expect him to propose to her via mail?

She was also evidently surprized that Lizzy was ignorant of the details surrounding Lydia's discovery. By her application to Lizzy for touring the Pemberley grounds, I think she considered them already engaged.