] Did this impact the timing of the proposal? I am toying with the idea that, if LC had been a truly genteel woman, well-bred and mannerly, Darcy would have been able to hold onto his Bennet-induced revulsion longer for the contrast, and might not have let his attraction to Lizzy run away with him and cause him to make that fateful trip to the parsonage.

I think Darcy could no longer resist -- Elizabeth was quite unlike any of the other women with whom he had socialized, and he was smitten. I suspect that he had put her out of his mind after he left Netherfield, but seeing her on a regular basis in Kent brought back all of her attractions and then some. As he says, "My feelings will not be repressed." He had reached a point where he simply had to make his declaration.