Although I appreciate your support of my take on the Bennet situation regarding Lydia's trip to Brighton, I have to disagree with your thought that Lizzy should have shown her father Darcy's letter. That would have been a disaster on so many levels.

First, showing anyone the letter (even Jane) would be an enormous breach of trust with Darcy. Since I have been convinced since the moment Darcy introduced Lizzy to Georgiana that he intends to propose again ("taking her to meet the family"). You don't introduce someone who's merely tolerable to meet your sister; you introduce someone who's special and expected to have an ongoing role in her life (kind of like the remarks Darcy made about Lizzy staying at Rosings Hall during future visits in Chapter 33). Darcy has trusted Lizzy with the letter; should Lizzy betray Darcy's trust (even before meeting Georgiana), she would be unworthy of his hand, should he offer it again.

Second, if the Gardiners' interest in Lizzy and Darcy was piqued by what they saw at Lambton and Pemberley to the extent that they assumed there was an understanding between Lizzy and Darcy, what would seeing a letter mean to Mr. Bennet, especially that letter, which criticized his own behavior and his family as well as suggesting at least the wish for an attachment with Lizzy.