] Still, Elinor, against her reason actually winds up forgiving him, amnd even feeling a teensy bit sorry for him ? Why ???!

Elinor began the confrontation with Willoughby full of anger and impatience. She asks him to leave several times.
As has been mentioned, Elinor was alone, emotionally exhausted, and her real concern must still be for Marieanne. Her mental energy to reason out his words must've been low.
Also, my impression is he wouldn't leave till she gave an assurance she thought better of him.
Yet that Charm thing kicked in-
'She felt that his influence over her mind was heightened by circumstances which ought not in reason to have weight; by that person of uncommon attraction- that open, affectionate, and lively manner which it was no merit to possess;'

Willoughby had behaved like a brother to Elinor more than John. It may partially account for Elinor's softened attitude,
'which made her think of him as now seperated for ever from her family with a tenderness, a regret, rather in proportion, she soon acknowledged, to his wishes than to his merits'.
(ch. 45)