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I just had a thought.
In episode five of Good Omens 2 Aziraphale reminds Shax his bookshop is a former outpost of heaven and therefore an embassy which they cannot enter. Shax retaliates by reminding HIM that he‘s an outcast, so technicalities don‘t exactly stand that firmly anymore. And while the demons were not actually able to enter the shop on those grounds, one of the shopkeepers did actually end up getting hurt (regardless of whether he was put back together or not).
I just thought, maybe Aziraphale was so willing to go back to heaven because he thought he‘d give humanity more security this way?
In the end, his decision to go back, to break Crowleys (and his own) heart was solely based on the hope to change things for the better - not for heaven or hell exactly if you ask me - but for humanity. He knew if he‘d run off with Crowley humans would‘ve been at the total mercy of heaven and hell and, let‘s face it, that‘s not the brightest future to have.
So while I absolutely agree he‘s beyond naïve in thinking he could actually change things, it‘s also the only choice he could‘ve realistically made. Because every other would‘ve meant to choose his own happiness over the survival of the world. And that would’ve just very much not been in character for him.