I agree with you in finding it hard to forgive Willoughby afer all his justifications. But obviously Elinor and Marianne donīt have that difficulty. In fact Elinor tells him:"Marianne does -- she has long forgiven you." I canīt remember whether we heard Marianne say such a thing - do you?

Elinor seems to be in a dither:
...which brought all the former Willoughby to her remembrance, and in spite of herself made her think him sincere.

...colouring likewise, and hardening her heart anew against any compassion for him,

..."Well, sir," said Elinor, who, though pitying him, grew impatient for his departure, "and this is all?"

Elinor's heart, which had undergone many changes in the course of this extraordinary conversation, was now softened again;

"You are very wrong, Mr. Willoughby, very blameable," said Elinor, while her voice, in spite of herself, betrayed her compassionate emotion;

Elinor assured him that she did(think the better of him); that she forgave, pitied, wished him well -- was even interested in his happiness