... he has seen all this, and if he thought about it at all, misinterpreted it.

First, I return to the time they lived in, and Lizzy's situation: a woman who was not well off could not remain a gentlewoman; she had to find work. Lizzy was likely to be poor. Almost any gentlewoman in Lizzy's situation would have married almost any well-established man. At least, this is Darcy's experience.

Second, We've seen continuing hints that Darcy is both less objectionable than Lizzy thinks, and that he's thoughtful, if not very understanding. Let's look at your objections.

But if he had:

1) seen Elizabeth's 'arch' manner bestowed on Colonel Fitzwilliam (probably more than on himself),

He would have seen Lizzy's charming response to an open-tempered conversationalist, charming himself. So Darcy's not open and chatty -- he'll have her mostly to himself, and will be in a position to win her over.

2) noticed her allowing Darcy to attempt all the approaches to her, and never trying to engage him herself in return,

She's being properly womanly, of course. Her lower station (keep in mind his opinion of their condition) requires her to wait for his advances. Recall her panic at hearing Collins's intent to address Darcy without introduction (ch. 18, at the Netherfield ball).

3) witnessed her withdrawal and discomfort caused by his references to her wanting to get away from Longbourne, to her staying at Rosings in future, and to his hints as to Charlotte marrying "well" (despite an unlikable husband),

Withdrawal and discomfort? She's modest and embarrassed at being trapped in isolation with a man who hasn't yet declared himself formally as a suitor! -- and saying such things! (when was a man lost in love ever rational? :-)

and still foresaw an unqualified agreement from her to the first open mention of romantic interest from him...!

Of course he doesn't see "unqualified agreement" -- he sees a societal trap, which he sees as inescapable, and which he's trying deliberately to exploit, after having spent several days (weeks?) trying to soften the insult by being as kind and accommodating as he is able.

Well, there is no other way to read that as o'erweening pride, is there?

Well, maybe. But in Charlotte's words (to which Lizzy playfully agreed, ch. 5), "If I may so express it, he has a right to be proud."