Interesting idea, but I'm not quite convinced by your argument, Barbara! ;-) As to the walking vs. driving argument, all I can say is that there is no mention of travel method in the second quote, which makes me think the narrator isn't talking about the difference between travelling by road or cutting across on foot. In the second quote, I get the impression that Anne is picturing half a mile as the crow flies.

Ch.1: Uppercross Cottage...was quite as likely to catch the traveller's eye, as the more consistent and considerable aspect and premises of the Great House, about a quarter of a mile farther on.

Ch.7: What was it to [Anne], if Frederick Wentworth were only half a mile distant, making himself agreeable to others?