Anghraine

I don't think he's shy, but I do think he's uncomfortable across a wide range of social situations in a way that e.g. Elizabeth and Colonel Fitzwilliam aren't (and that it's easy but not quite fair for two very outgoing people to dismiss temperamental difference altogether, even though their fundamental point is valid). I take him at his word that he's uneasy in unfamiliar social situations—and right to the end of the book, we see that.

After all, reformed Darcy at Longbourn is not very different from original Darcy at Longbourn. Darcy at Pemberley acts the way we've just heard he always acts at Pemberley. But Darcy also acts the way he does at Pemberley when he visits the Gardiners in Gracechurch Street—I don't think he would have contemplated the idea that a merchant's house in Gracechurch Street could be comfortable, before. There's a willingness to extend that self beyond his private world now, even if he can't do it everywhere (as with the hilarious failboat he is back in Hertfordshire).

I think Darcy's social discomfort is also suggested by the pretty drastic change in Elizabeth's perspective after their engagement, when she tries to shield him from people he isn't comfortable with. Honestly, I would find that element rather unsettling if there weren't a difference in social ability there. But as is, I think it's rather sweet that she goes from completely rejecting temperament to vaguely protective as long as he's putting in his best effort. Sometimes Darcy's best is not going to be very good, and she's okay with that in a way she wouldn't have been before. But Darcy's "I'm too special to make any effort except when it isn't really much effort" was ... ah, not okay. And not shyness.

When most people pass him off as shy, it comes too close to acquittal, which unbalances the story

Hmm. I do see that argument quite a lot, but I don't think shyness and pride are mutually incompatible. He's forceful and confident, not shy, but a shy!Darcy would still be a snobbish asshole. The issue is when he's read as shy instead of proud rather than when he's read as shy and proud (though I agree with neither).

(Aside: I do really, really like The Fabric of Dialogue, and I think it's available online.)