] 1) I'm wondering about the use of "down" in relation to geography.
I think it has to do with the starting point of the person who travelles. If someone is going away from their home they are going down. Miss King lived in Meryton but moved to her uncle. Miss Darcy has an establishment in London and visited Pemberley. My parents used to explain this to me as when we went into a store we would go down to the end and then back up to the front, the starting point the entrance of the store.
]4) Mrs. Gardiner "had lately learned that some acquaintances still remained" in Lambton.
Wickham had told her about her friends in chapter 25. He might be the source she is referring to.