Upon reading Willoughby's maudlin musings about Barton Cottage in Chapter 14, I thought to myself, this sounds like a farewell speech. In fact, I thought it very much resembled Marianne's histrionic farewell to Norland in Chapter 5...Stay the same, you will never change in my memory, etc. etc. " (paraphrasing).

But he makes an appointment to visit the Dashwood ladies at 4 the next evening. Mrs. Dashwood allows Marianne to beg off the visit to Lady Middleton because she thinks perhaps Willoughby and Marianne have arranged for him to be able to see her (Marianne) alone. And it turns out Mrs. Dashwood is right...Willoughby HAS come to see Marianne, but the purpose of his visit is to announce that he has to leave immediately.

I think Willougby knew he was leaving the day he made all his tenderhearted speeches about the Dashwoods and Barton Cottage.

I think he DID prearrange to come see Marianne alone.

I think his purpose in seeing her alone was to avoid Mrs. Dashwood and Elinor's questions about his leaving. Unfortunately for him, they returned before he could make his escape.