By irrational, I mean that there's no need for Darcy's behaviour to be consistent on every point, and that I don't expect him to have proceeded like the "reasonable person" that law courts delight in imagining. I'm not meaning that he's hallucinating or psychotic.

I think Darcy *may* have talked to Colonel Fitzwilliam, but there's no sign that he did. And even if you think it's the most reasonable thing in the world that he would talk to someone about his marriage, I don't think there's any reason to believe that Darcy is capable of being reasonable about Elizabeth. To quote Buffy the Vampireslayer "Love makes you do the whacky".