yes interesting question indeed Barbara.

I think despite Marianne's insistance against exertion, she'd involuntarily try to moderate herself in front of Mrs Jennings for fear of another ill-timed comment or proposition, whereas with her mother, it'd surely be fuel in the fire. 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