] Could his visit have lasted 3 - 4 months (Sept - Dec), with Willoughby leaving for London in January?

He was definitely there from early November through Christmas, as he tells Marianne something about 'this time last year' or similar. But even if he were there as early as the previous September, he's still back much, much less than twelve months later and the statement 'never repeated within a twelvemonth' is recognizably untrue. He could as easily have said 'I come but once a year', but I don't think Jane Austen's word choice is a coincidence. I also think she quite carefully laid the groundwork to let us know that this was not a true statement.