] she actually "could not refuse to give him hers" when he holds out his hand in farewell. I, for one, would have crossed my arms firmly over my chest,

Are you certain? I'm not sure I could, though I thoroughly despise Willoughby in this scene. This, for me, is one of the most chilling passages in Austen, simply because it points out the power of such charm even over such a levelheaded person as Elinor. If she could not refuse, I doubt seriously whether I could.

]but I think it's easier for us readers to judge W as he deserves, than for someone in his devilishly charming presence.

Exactly. I've known people who were unreliable, outright liars, sources of pain to all those about them, who were nevertheless so attractive in person that all these considerations dimmed in their presence. In the company of such magnetic attraction, one must be very strong-minded indeed to resist. It says a lot for Elinor that she can be as firm with him as she does manage to be in this scene.