Farther down the board, there was quite a bit of discussion about whether Willoughby had made Sophia believe he was almost or secretly engaged to her, in the way that he made Marianne (and probably also Eliza) believe that the engagement (or that he loved her) 'was every day implied, but never professedly declared. '

Sophia's guardian Mrs. Ellison's comment that the engagement 'was no longer to be a secret' fits in perfectly with Willoughby's claim that "I had reason to believe myself secure of my present wife, if I chose to address her". If he had done everything but actually speak the words 'Will you marry me?'--if he had left Sophia believing he would come back, believing he loved her, believing he was just on the verge of asking him to marry her (as he had done with Marianne), then he probably did feel pretty sure of himself. And it's not like he hasn't got experience with this type of thing.

And how many other girls? I agree that Eliza was not the first or the only one before Sophia and Marianne. When he says 'because I was a libertine, she must be a saint', that really doesn't sound like his seduction of Eliza was the first or only time he had done that.