Each of the three men has affection for one of the Miss Dashwoods and it seems we are to be in suspense about them and if any one of them will come though for the dear ladies. I do not think Col Brandon’s departure had much affect on the Dashwoods. In Ch. 14 Elinor more or less dismissed the dear colonel’s departure as of little interest:

Elinor, though she felt really interested in the welfare of Colonel Brandon, could not bestow all the wonder on his going so suddenly away, which Mrs. Jennings was desirous of her feeling; for besides that the circumstance did not in her opinion justify such lasting amazement or variety of speculation, her wonder was otherwise disposed of. …Why they should not openly acknowledge to her mother and herself, what their constant behaviour to each other declared to have taken place, Elinor could not imagine. (Ch. 14)

I think Willoughby’s departure is so emotional because everyone, including Dear Doubting Elinor, believes his behavior to Marianne is that of an engaged man. When he leaves without a formal declaration or an intent of returning and refuses to accept Mrs. Dashwoods invitation they are all flabbergasted by his behavior. Edward is moody, sometimes low and his leaving without knowing where he is going is downright strange but the Dashwoods think they understand the cause. They believe his lowness is due to the strife between him and his family. Mrs. Ferrars is to blame! (:D) I agree with MarianneR (39969) that Edward’s arrival did gentle the after affects of Willoughby’s departure, even for Marianne:

“He was the only person in the world who could at that moment be forgiven for not being Willoughby; the only one who could have gained a smile from her; but she dispersed her tears to smile on him , and in her sister's happiness forgot for a time her own disappointment.” (Ch. 16)