You’ve nailed it though, OP. Aziraphale mishandled the whole thing because he did not anticipate how Crowley would react, even though the entire audience did. And I think that comes down to the same reason he struggles with accepting the truth about Heaven: his truly impressive powers of self-delusion.
Aziraphale has been living with a massive cognitive dissonance as long as Crowley has been living with his trauma. He’s coped mostly by not coping at all, by convincing himself it all must make sense somehow. The Ineffable Plan (specifically the ineffability of it) is a comforting thought to him. Crowley hates not knowing, but Aziraphale prefers not knowing. Not knowing means he can tell himself there is a good reason, even if he doesn’t understand it.
Anyway, so he’s really really good at lying to himself and very selective about what he notices. And that’s a serious hindrance in his relationship to Crowley.
Because Aziraphale is well aware of their fundamental differences in belief, but he is deluding himself about what causes them.
He’s still holding on to the view that he’s right. He has a patronizing view of Crowley at times, alternately dismissing him (‘well, of course not, you’re a demon’) and acting So Pleased when he turns out to actually be an alright chap. All well and fine, he’s an angel, the condescension’s part of the package. But there’s a deeper problem, because Aziraphale only associates good with Heaven. He thinks the reason Crowley is good is because he’s still an angel deep down.
That’s it. That’s really it. Crowley isn’t really all that evil, therefore Crowley is still partly aligned with Heaven. Aziraphale says as much in the Job flashback. Crowley denies it. But Crowley denies a lot of things. I think Aziraphale waved that off. I think he’s still holding on to that perception in the present day. It’s simple logic, and it doesn’t require his own worldview to twist all that much. In his mind, Crowley is still mostly an angel deep down. Throwing him out of Heaven was a mistake, not because it was a horrible thing to do to anyone, but because Crowley didn’t deserve it.
He still hasn’t decoupled the idea of good from Heaven. The mean Archangels are ‘bad angels’. Crowley, by the same logic, is a good angel. Why shouldn’t he be restored to angel status? Heaven is where the good people belong.
As long as Aziraphale thinks like that, he cannot truly understand why Crowley wouldn’t want to return to Heaven. He can’t understand that restoring him as an angel wouldn’t make him happy. Aziraphale genuinely thought that it would. And Crowley’s just been doused with the horrible reality that not only has Aziraphale not been de-programmed, not only is he still holding that view, he also thinks he can still fit Crowley into that view. After all this time knowing him. Aziraphale isn’t stupid, it’s not that he doesn’t pay attention, it’s that he doesn’t see what he doesn’t want to see. He doesn’t want to look truth in its slitted yellow eyes.
And Crowley can’t keep trying to get through to him. He can’t do this anymore. His confession landed on deaf ears because Aziraphale was too busy telling him horrible news with a big smile on his face. Crowley’s patience and optimism has finally broken. He’s done. He still loves Aziraphale, he’ll never not love him, but he’s finally done trying to save him from himself.
And that is what will finally allow Aziraphale’s full character potential to be realized. Because he’s going to have to confront his delusions and test his assumptions by himself. Crowley’s spent 6,000 years trying to open his eyes, but they won’t properly open until Aziraphale decides to open them.
