]What's more, while Elinor can work her magic with her mother via letters, Mrs Dashwood would certainly have a different opinion of Willoughby's conduct had she been there herself. If Elinor pressed her case in person, Mrs Dashwood would simply continue to reject her claims as being cold-hearted towards a dear young man :P

Are you saying that Mrs Dashwood wouldn't be as likely to believe Elinor in person? I quite disagree: we must give her more credit for understanding the situation. Had Mrs D. been there to see Marianne's pain in person, she probably would have reacted with even more disappointment and indignation. Mrs Dashwood liked Willoughby, true, but mainly because Marianne loved him and he seemed to reciprocate. Her love for Marianne is stronger than any affection for Willoughby, though; she would hardly take his side against her daughter's, especially when Marianne is so miserable.