It is exactly by your sort of reasoning, Belinda, that I must believe Tolkien is being metaphorical; in other words, a darkness so oppressive as if it is a thing with being of its own rather than just an absence of light. My interpretation of Melkor is that he cannot create; only corrupt. He can take Light and maliciously corrupt it into darkness, but he cannot bring a thing into being separate from creation.In Chapter 8 of The Silmarillion Tolkien wrote:
So the great darkness fell upon Valinor.---------------------------------------------The Light failed: but the Darkness that followed was more than loss of light, In that hour was made a Darkness that seemed not a lack but a thing with being ofts own : for it was indeed made by malice out of Light-------
Tolkien is saying that evil is not a necessary part of creation but is a substance separate from creation. Although this point of view may inspire and encourage us to fight evil, it cannot be the case because evil is what is against life. Death is an evil, destructive natural events are evils. An evil man is evil because antisocial, he fails to harmonise with life which, practically speaking,involves man in societies.
Although I balk at calling Melkor a creator, I ultimately agree with you that Tolkien sheds no new light on the problem of evil. He is trapped by either (1) giving Evil independent ontology from Good which makes it impossible to examine except metaphorically (hence your thoughts about a fictional narrative); or (2) Evil's ultimate genesis is from Good, which really muddies the philosophical waters, generating the problem of evil all over again.Tolkien's use of 'malice' is interesting because it implies that Tolkien views malice as able to exist without an object of malice; except insofar as malice sets itself against the Light, i.e. the creation. Since the Valar and the rest of Eru's creation are good apart from Melkor the evil one, Melkor is against the creation and , to allow a point, the Creator . This scenario cannot come about without some narrative such as Genesis or The Silmarillion to explain it. But Genesis and The Silmarillion are fictions which explain the birth of evil. So Tolkien has made a circular argument for the independent existence of evil.
Giving Evil independent ontology from Good has historical precedent in neo-orthodox 5th century A.D. Zoroastrianism which posited a pure dualism between Good and Evil as irreconcilable ontological primaries. Despite the marvelously clear distinction this makes between good and evil in human lives (which Zoroastrian doctrine takes full advantage of), we are left with no way to understand philosophicallly how good and evil can even be recognized in such a scheme, let alone interact. Yes Zoroastrian scripture assigns virtues and vices in accord with what we might expect, but offers no explanations for how such evaluation isn't arbitrary, except to assign omniscience and creativity to the Good God (Ahura Mazda), but ignorance and destructiveness to the Evil God (Aingra Manyu). Armed with omniscience, Ahura Mazda knows all of Aingra Mainyu's designs in advance and can build fortification against them and also turn destruction into a tool for subsequent creation. (Sounds like Eru proclaiming Melkor's corruption as being for the greater glory of Arda).
The Zoroastrian view is ultimately undermined when one realizes what you realized: the scripture cannot act as an independent verifier of good and evil without the whole viewpoint becoming circular.