I'm in the camp that believes the idea of proposing to Elizabeth did not just suddenly come to Darcy when he went to her at the parsonage. From sometime at or after the piano scene at Rosings, I believe he was not only practicing, but perhaps -- in his mind anyway -- courting Elizabeth. Again I refer back to chapter 12 -- when Darcy was "careful that no sign of his admiration should now escape him, nothing that could elevate her with the hope of influencing his felicity . . .if such an idea had been suggested." If he thought that his admiration for Elizabeth might have been apparent to her at Netherfield, just think how successful he must have felt his "courtship" was going: she suggested he "practice" conversing! She seemed not disturbed by the prospect of living away from her family! She told him her favorite walks!

And, he did already postponed his departure once. We learn this from Colonel Fitzwilliam in Ch 33, and while there is no reason given, I cannot imagine that Darcy put off his departure for any other reason than to start or continue his "courtship."