About The Metatron's motivation to promote Aziraphale...
...why people usually dismiss this part?
Unlike other The Metatron’s ‘compliments’ made as an ‘argument’ to why Aziraphale should be a new Supreme Archangel, this one is made in later part of the conversation (not in the French restaurant, but right before the elevator, and right before ‘we call it the Second Coming’ bombshell). Unlike those ‘compliments’ this one isn’t a blatant lie – Aziraphale is familiar with ‘Earth ways’, not perfectly, but better than most angels.
And, unlike The Metatron’s earlier reasoning, this one can be true (or at least part of the truth).
Heaven are preparing a new Armageddon. The last one attempt was prevented mostly by humans, with aziracrow acting as a support at best. If The Metatron researched it properly to avoid mistakes of Hell, he should know about that. So humans of Earth might be considered as a significant threat to a Great Plan* - probably more significant than two defectors, even if these two are somehow powerful (if The Metatron is even aware about their 25 Lazarii miracle).
(*also, remember: The Big One would be All of Us against all of Them, with Them meaning Humanity)
There might be a reason for The Metatron to start to go on Earth and “blend in”, to the point of understanding how to order coffee and have a small talk with a barista: to understand the enemy in general.
And tell me, isn’t Aziraphale an angel who spent more time on Earth than any other angel - literally all of its 6000 years? Doesn't he have the most experience with humans and their antics? Wasn’t he there when humans ruined Hell’s plans of Armageddon, meaning he should (at least in theory) understand how they did it and for what would a half-human Messiah reject their purpose?
And he is smart. Wouldn’t it be a waste to not use this source of knowledge for the benefit of Heaven? Of course, this one angel won’t just agree to work with Heaven again (we heard him refusing a few times in S2E6), so there must be something that will convince him – this part, how he was convinced, was discussed a lot of times and isn’t the point of this post, though.
I think, it also can lead to Aziraphale’s memories not being wiped in S3 and him having some (if very limited) capacity to act on his own. If The Metatron’s main purpose is just divide aziracrow or something like that, then wiping angel’s memories looks like the most logical action of his part. But if his main purpose is using Aziraphale's knowledge and experience, if angel’s memories are the main reason of why he is needed on Heaven – then memory wiping doesn’t look like a very good idea anymore.