Great insights, Robbin. I think your initial quote can be boiled down to Willoughby hearing Elinor say loud and clear "What are you doing here?!"

As soon as Willoughby heard this, he apparently chose to adopt the direct - and perhaps bullying - course because he knew she was impervious to his charms. She was ready to leave, but he drew her back by insisting that he had business with her. It's funny though that his declaration for the remainder of the chapter has nothing to do with Elinor. His business actually did not concern her except as she was the sister of Marianne.

Willoughby's a jerk to the end. It's too bad Mr. Palmer wasn't there. He might have been able to show Elinor where they kept the horsewhips.