I like to analyze what D&D alignments fictional and historical characters might be as much as any D&D fan, because it's fun, but I think all this trying to shoehorn Christian theology and cosmology into D&D cosmology is in error and misguided.
Christianity is an extremely dualistic religion - either Good OR Evil, no in-between, either God OR Satan; and monotheistic - Satan only exists or has any power because God allows it. There is no in-between other than perhaps the Catholic idea of Purgatory, and Protestants are going to have a problem with that, i.e. there's no such thing as Purgatory to a Protestant theologian or believer. There is no God but God. Other gods do not exist - they are idolatrous made-up creations of misguided and evil humans.
D&D is polytheistic in its fantasy religion and moral system to the core. Many gods representing many alignments demonstrably exist and grant favor and power to their followers who please them. It's fantasy. (I'd say all real-life religion is fantasy, too, of a different kind, but that's another discussion.)
Christianity is an either-or religion, not a spectrum of possible alignments. Either you are a follower of Christ and depend on Him for your salvation into Heaven, or you are doomed to Hell. Following and believing in Christ and His grace is Good. Not following Christ, or phrased the other way around, following and believing in anything but Christ, makes you Evil, because all humans are Evil by nature and by default, ever since the Fall, and only Christ can save us, change our base natures, and bring us to God's Goodness, from which we are separated.
Fantasy religion and morality are interesting, but I don't think we can try to apply them to real-life theology, philosophy, ethics, or character analysis and expect to be taken seriously.