I'm sorry after you just said that was your last word to reply to this, but I swear I have a new point to make. Hopefully it's not one that has been made before :-)

One thing that seems to have been lacking was proper adult guidance for Marianne, Eliza, Lydia, and Georgiana- all young girls who are foolish to varying extents. There was a long discussion already on the board of the failure of Mrs. Dashwood to properly oversee Marianne's relationship with Willoughby. I believe this supports my point that the "age of consent" has nothing to do with a girl's being ready to make decisions to be sexually active. If Eliza and Lydia were fully of age to know better and decide to have sex, then why should they need any kind of adult guidance? Why would Mrs. Dashwood have to ask "What are your intentions?" if Marianne was old enough to decide for herself. Perhaps there's another discrepancy. Was Marianne was old enough to make decisions about sex, but not of age to make decisions about marriage?

I've seen enough Marianne bashing on the S&S board over the years to know that few (if any) readers find her faultless. Her own romantic notions and her immaturity too contributed to her disappointment. She, thank goodness, wizened up when she reflected during her illness. Her plan of making amends was perhaps as eager in the other direction, but that is just her nature.

As far as Willoughby's bad motives, I agree that it is not necessary to apply them to everything he does. His confession speaks for itself. He complains that Col. Brandon might present him as capable of anything. Col. Brandon didn't have to lie to prove this, Willoughby's behavior speaks for itself. It's bad enough as it is. I also don't believe W was lying in his confession. If he was, surely he'd make himself sound better than he did. When pressed, I think he is a bad liar, and skirts rather than have to answer directly.
ch. 15
"My engagements at present," replied Willoughby confusedly, "are of such a nature -- that -- I dare not flatter myself" --

He stopt. Mrs. Dashwood was too much astonished to speak, and another pause succeeded. This was broken by Willoughby, who said with a faint smile, "It is folly to linger in this manner. I will not torment myself any longer by remaining among friends whose society it is impossible for me now to enjoy."

and another in ch. 44
"Did you tell her that you should soon return?"

"I do not know what I told her," he replied, impatiently; "less than was due to the past, beyond a doubt, and in all likelihood much more than was justified by the future. I cannot think of it -- it won't do.

I think the most outright lie in the confession or close to it is this
You must have known, that while you were enjoying yourself in Devonshire, pursuing fresh schemes, always gay, always happy, she was reduced to the extremest indigence."

"But, upon my soul, I did not know it," he warmly replied; "I did not recollect that I had omitted to give her my direction; and common-sense might have told her how to find it out."
He seems sincere in his warm reply, but it seems mighty convenient that he forgot to tell her how to find him. If this is the utter truth, he still exposes his guilt, because he seems not to have offered any relief once he did find out about her situation. He seems to be perfectly content (as far as we know) to ignore the existence of his own child. So if he was telling the truth in that sentence, he is conveniently omitting that he has done nothing for Eliza or the child since finding out. He only reveals to justify himself. As others have pointed out, he is very human. Austen was tricky in that she engages our sympathies by our recognition "Oh look he has feelings." I admit I used to feel more sympathy for him, but heart hardens further against him with each re-read. His feelings amount to bumpkis, if he cares nothing about the feelings of others.

Regardless of the letter of the law, Col. Brandon found his crime so horrid, that knowing Willoughby was going to get off scot free, he sought to punish him in the only way available to him, a duel.

I think Elinor has the right of it when she says to Willoughby
"Do not think yourself excused by any weakness, any natural defect of understanding on her side, in the wanton cruelty so evident on yours."