Well I'm avoiding Mothers' Day preparations in the US Midwest, so I'll have a go...
This one nags at me every time I read P&P. Was there a Bennet back history in Jane's mind that did not make it into print? Is the statement hanging from a bit of plot that was revised out of the text? Is Mr. Bennet a younger son, or even a cousin, who unexpectedly inherited? But the Bennet daughters must always have been at Longbourn. We know that Mr. & Mrs. Bennet expected to have a son to break the entail, and should have been setting aside income for the girls' dowries. I think it's reasonable to assume Mr. Bennet was already in possession of the estate when his daughters were born. Does Darcy just assume the family has been doing the usual London-for-the-season jaunt? To my mind "You cannot always have been at Longbourn!" implies something about the ownership of the estate. Darcy probably doesn't feel spending the season in London makes him less attached to Pemberley.
I believe everyone in Hertfordshire would know the Bennet backstory. Four and twenty families, with Mrs. Bennet and Mrs. Lucas in their number, cannot have many secrets from one another.
The lack of question marks I quite understand. When Fitzwilliam Darcy supposes aloud, he is not asking a question. He fully expects reality to meet his supposition. But even with a question mark, "You cannot have always been at Longbourn?" implies Darcy has a bit of information that we, to my great irritation, do not possess.
