) I think his goal is some sort of dramatic presentation of how far he's come in his agony and pain over Marieanne.

Willoughby plays out a romantic hero for himself and Elinor.
Yet, in a more ordinary way after a long melancholy walk and damp stockings, Marieanne develops an infectious bad cold.
It gratified Willoughby's vanity to think Marieanne must be dying of love for him,
' Yet when I thought of her today as really dying, it was a kind of comfort to me to imagine that I knew exactly how she would appear to those who saw her last in this world.'

He does seem to revel in the drama. He is also concerned to make himself look better to the Dashwoods. Yet Willoughby is quite clear, '...had I really loved, could I have, sacrified my own feelings to vanity, to avarice--or, what is more, could I have sacrified hers-But I have done it'.
At most, his confession says he does not love Marieanne and he treated her badly. Yet still, he must play up a drama of his own pain and suffering.

I also agree on Robbin's point if Willoughby thought Mr Palmer was at Cleveland he wouldn't have come. Imagine his quick retreat if he's run into Col. Brandon.

What I find almost unbelieveable is him admitting, 'I endeavoured by every means in my power, to make myself pleasing to her, without any design of returning her affection.'