I think Willoughby might be putting on a bit of a show for Elinor in Ch. 44. After Elinor says Mr. Palmer is not at home Willoughby says:

"Had they told me," he cried with vehemence, "that Mr. Palmer and all his relations were at the devil, it would not have turned me from the door. My business is with you, and only you."

The thing is though, Willoughby knew before he left London that Mr. Palmer was not at home. He learned from Sir John that “Marianne Dashwood was dying of a putrid fever at Cleveland …the Palmers all gone off in a fright” He knew very well neither Mr. Palmer nor all his relations would be at Cleveland to turn him from the door which is exactly what would have happened had Mr. Palmer or Col Brandon had been there. Willoughby is no friend to either of these men and even Mr. Palmer has showed concern for Elinor and Marianne since Willoughby’s treachery. I do not suppose Willoughby suspected Col Brandon to be around. I think Willoughby expected to find Elinor & Marianne alone without any male protection and that is just the way he likes it.

Is Willoughby trying to cast his visit to Cleveland as a grand gesture to his dying love? When Elinor suspects Willoughby might be drunk he agrees and tells her of his arduous journey of eleven hours and fifty minutes in a carriage without break, how he ate little and thus became drunk on just a pint:

Elinor looked at him with greater astonishment than ever. She began to think that he must be in liquor; -- the strangeness of such a visit, and of such manners, seemed no otherwise intelligible; and with this impression she immediately rose, saying –

"Mr. Willoughby, I advise you at present to return to Combe. I am not at leisure to remain with you longer. Whatever your business may be with me, it will be better recollected and explained to-morrow."

"I understand you," he replied, with an expressive smile, and a voice perfectly calm. "Yes, I am very drunk. -- A pint of porter with my cold beef at Marlborough was enough to overset me."

"At Marlborough!" cried Elinor, more and more at a loss to understand what he would be at.

"Yes -- I left London this morning at eight o'clock, and the only ten minutes I have spent out of my chaise since that time, procured me a nuncheon at Marlborough."

The steadiness of his manner, and the intelligence of his eye as he spoke, convincing Elinor, that whatever other unpardonable folly might bring him to Cleveland, he was not brought there by intoxication, she said, after a moment's recollection,