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'What if God's plan is really to get them together?'
'What if God actually wants the angels and demons to think for themselves?'
'What if God actually wants the Earth to be saved?'
These questions are missing the entire point of the story.
I cannot stress enough that this is a satire of Christian fundamentalism written by an atheist and a Jew. What God really wants or has planned is not remotely the point. God could be dead for all that it matters. We're never going to know more about God. And we're not supposed to base our notion of what's right on what God wants. That's what fundamentalism does. Good Omens is specifically subverting that.
The only real answer to these questions is 'It doesn't fucking matter.' We're talking about right and wrong, good and bad, as it affects people (human and otherwise). We have to decide what happens. God doesn't get a say. Maybe they'll approve, maybe they won't. Fuck 'em either way.