Let's do talk about Elinor's reflections.

According to the Ellen Moody calendar, the Willoughby visit was on April 16th.
ch. 45 In the time after Willoughby leaves, before her mother arrives, she regrets his separation from her family rather in proportion, as she soon acknowledged within herself to his wishes than to his merits.

heightened by circumstances which ought not in reason to have weight; ...by that still ardent love for Marianne, which it was not even innocent to indulge

Elinor is worn out and weakened. At this point she calls it ardent love in her mind. When she rests and reflects does she no longer consider it love? Tricky tricky Jane.

Again according to the calendar on April 30th, two weeks later(in ch. 46) Elinor tells Marianne:
related simply and honestly the chief points on which Willoughby grounded his apology; did justice to his repentance, and softened only his protestations of present regard.

In Elinor's mind it was an apology and repentance. But as Robbin pointed out below, careful study of Ch.44 reveals that he didn't really say "I'm sorry" and all of his expressions of regret seem to include a deflection of his guilt. His self-flagellation has inaccurate aim because the whip seems to miss at the last second to lash out towards someone else. His feelings are sincere, but they seem to be all for himself. Again I say tricky tricky Jane.

That same evening, Elinor tells the story to her mother (ch 47):
Reflection had given calmness to her judgment, and sobered her own opinion of Willoughby's deserts; she wished, therefore, to declare only the simple truth, and lay open such facts as were really due to his character,

So I really do I wonder what exactly this truth was that she relayed to Marianne and Mrs. Dashwood. She probably told them that Willoughby had some affection for Marianne, maybe that the letter wasn't his own words, and that he married Miss Grey for money. It says with Marianne she was carefully minute in every particular of speech and look, where minuteness could be safely indulged. It seems to me that in every minute confession of his there is some equally d&%ning revelation of his character. What does everyone else think Elinor said?

Mrs. Dashwood did not hear unmoved the vindication of her former favourite. She rejoiced in his being cleared from some part of his imputed guilt;

Vindication? Cleared? The only "some part" I can think here is yes, there was some affection for Marianne, and if only for an instant he wanted to marry her.

More of Mrs. Dashwood
But the feelings of the past could not be recalled. Nothing could restore him with a faith unbroken -- a character unblemished, to Marianne. Nothing could do away the knowledge of what the latter had suffered through his means, nor remove the guilt of his conduct towards Eliza.

Elinor did not want to rouse more feelings of compassion towards Willoughby, like those which were called forth in herself... because sober reflection shows that this above paragraph for Mrs. Dashwood was a proper assessment of the situation (at least in my opinion).

Though he probably didn't deserve it, I can understand why Elinor forgave him (religious perspective) for the Marianne business. But I'm sure if he ever pressed himself on her family's notice again, she would retract it.

His so called repentance was so utterly inadequate(and much of it rather icky). True repentance would mean trying to make things right where he could. I am sure that the Col. would never want him to contact Eliza again, but it seems he could have done something to support his child, by sending money to the Col. to give to her. He said Domestic happiness is out of the question.. He chose his wife and is not willing to do what it takes to make his marriage a good one.

I do think that Jane Austen was rather tricky. We get Willoughby's confession, full of feeling (on study we see it's all for himself), but we also get the contrast with Colonel Brandon, a man of true character. The Col. is a man who truly tries to relieve others' sufferings. Is this another instance of sense vs. sensibility, or both? On first thought we see Willoughby's feelings(sensibility), but also Willoughby has a selfish sense that protects himself from getting too involved, keeps him out of legal engagements, cunningly uses and throws away a young girl, marries very deliberately for money.

The Col. is a man of feeling and of good sense. We can compare Willoughby to Colonel Brandon and see that the Col. is a better man in every way!

I apologize for the rambling nature of this post. I do wonder what Elinor would think. I also wonder if Austen ever imagined that we'd be inspecting every word so carefully centuries later.