]Interesting that Elizabeth would expect the Darcy siblings to be so alike, when her own siblings are so different.

Interesting question. A residue of Wickham's influence, probably. After all, he had been right about Lady C! Finding Georgiana the exact opposite obviously relieved her, and also made her realize even more, what a jerk Wickham was!

She says to the Gardiners later that '...his lies about the whole Pemberley family are endless. From what he said of Miss Darcy, I was thoroughly prepared to see a proud, reserved, disagreeable girl. Yet he knew to the contrary himself.'

]Bingley didn't come with the Darcys? Why not?

He probably rode a horse to the inn, leaving the Darcy's to arrive in the curricle.
I checked the 'JaneInfo' page, and it defines a curricle as- Curricles and gigs are light two-wheeled carriages open in front. They seat no more than two people (one of whom acts as the driver)

]Does this mean they slept in the carriage? While it continued driving south through the night? Sounds rather uncomfortable, even dangerous. And the poor coachman!

I took this mean that they had slept in an inn. I might be wrong :)

]Kitty says "I would behave better than Lydia". Does this mean she realizes that Lydia has behaved reprehensibly?

I think she was almost as silly as Lydia, but would not dare to do elope...
Moreover, I have a feeling that she did not suceed in attracting either offciers, or trouble as Lydia did!