I don't think Darcy's decision to propose to Elizabeth was a rational choice. His wording sounds extremely like he was driven to propose about everything his reason told him:

"In vain have I struggled. It will not do. My feelings will not be repressed. You must allow me to tell you how ardently I admire and love you."
...
He concluded with representing to her the strength of that attachment which, in spite of all his endeavours, he had found impossible to conquer; and with expressing his hope that it would now be rewarded by her acceptance of his hand. (chpt 34)

From these lines, I see Darcy as simply not being able to bear the thought of living without Elizabeth. We don't know exactly what the thoughts are running through his mind, but for whatever reason, he can't help himself, he loves her, he wants her making witty comments at the head of the dining table for the rest of his life. Darcy's brain is telling him that marrying Elizabeth would be a degradation, his heart is telling him that he must have her. In this situation friendly objective reflections would be useless, no matter how sympathetic the confidant.

And I think this also explains why Darcy doesn't consider that he has very little clue about Elizabeth's feelings - he's too caught up in his own struggle about his own feelings. And then he's under time pressure. He's shortly due to leave Rosings, when he does he has no reason to believe he'll ever see Elizabeth again, this is his chance.