Everything you say of Brandon and Willoughby is spot on. (:D) I am not sure Willoughby would have even come to them and spoken to Elinor had not Marianne called his name and held out her hand. I think he might have been hoping they would let him mosey off with his lady friend:

At last he turned round again, and regarded them both; she started up, and pronouncing his name in a tone of affection, held out her hand to him. He approached, and addressing himself rather to Elinor than Marianne, as if wishing to avoid her eye, and determined not to observe her attitude, (Ch. 28)