I think it quite natural that Harville and Wentworth should fall into conversation with each other. They are especially close friends. As to Anne ending up near Benwick -- I'm not sure I understand how that is neglect. Benwick is considered quite as a member of the family in the Harville household. (Captain Harville himself states that "[the Harvilles and Benwick] cannot part" [ch. 12].) And earlier in that same paragraph, Captain Harville makes a point of telling Anne that her kindness toward Benwick has been very good for him.