I'm not sure she would have been unhappy with an actual excursion to Whitwell, and she might have had a favourable anticipation of that excursion IF the weather had looked more favourable.

Marianne's & Mrs Dashwood's expectations of happiness may be more fantasy than realistic expectation, imo. Marianne's expectation of seeing Willoughby in London probably included visions of spending nearly as much time with him as she had been used to at Barton -- and that would not be realistic, imo, even if Willoughby had actually loved her.

Perhaps the healthier outlook would be somewhere in between these two points of view? Maybe Elinor is a little too realistic, a little too negative as a counterweight to her mother & sister?