When will Darcy have a chance to 'win her over'? After the marriage he is proposing?

Yes. When he's able to show her his love, that he can accept her, if not her relations and their conduct, that he truly is more than he can express in words in front of strangers.

Then it will not hold that this plan helped raise his confidence in a positive response to that idea.

Why do you think he's planning this? He understands their respective situations; he knows what will almost certainly happen; he's not thinking beyond that (imo), when all his desperation bursts forth in a clearly unplanned proposal. IMO.

I think I can see your take on this episode in individual pieces, but when you put them together, you seem to be saying (please, correct me if I mistake you) that sometimes he thinks she is interested-but-demure, and sometimes that she will have to accept him, despite having no current affection for him.

Well, no; I don't think at all that he's looking at her as interested-but-demure; I think he's looking at her as trapped by their society and its rules. I do believe firmly that he has hopes, even confidence of being able to win her over. It doesn't appear to me that he devotes the least attention to her present feelings toward him -- which of course, is a complete justification of her opinion of him.

I hope Darcy is not exploiting a societal trap - I hope he sincerely believes (in pride and arrogance, 'tis true!) that Elizabeth LIKES him. Then all of his attitudes come from blindness to her nuances in pursuit of his main goal.