How do you see the natures of Eliza and Marianne figuring into Willoughby’s choices except as reasons to take advantage of them as you explained? If Willoughby believed seducing the natural daughter of “someone” would be less egregious than seducing a more respectable Marianne then it strengthens the argument that he is a predator. If he chose Eliza based on the fact she has low status and little protection then he is cutting her out of the herd for seduction because of those weaknesses. The same is true if he chooses girls who are impulsive and romantic because they are easier to seduce.

He had left the girl whose youth and innocence he had seduced, in a situation of the utmost distress, with no creditable home, no help, no friends, ignorant of his address! He had left her, promising to return; he neither returned, nor wrote, nor relieved her." (Ch. 31)

Eliza could have said no to Willoughby but he had that option as well. He might have determined not to take advantage of a teenage girl. No one forced him to behave badly. The object of a seduction is to create a willing sexual partner so the fact Eliza was willing does not mean she was not seduced. I agree Eliza may be impulsive and romantic but I do not see any evidence that she was cruising Bath for a physical relationship anymore than Marianne was tumbling down hill looking for one. Is there any reason to doubt Col Brandon’s description (above) of Willoughby seducing a young and innocent Eliza?