I want to outline why I, personally, do not favour the “unreliable narrator” theory about The Silmarillion. I’ve heard two versions of it, one with the narration being biased against the Fëanorians, the other with it being biased against the Sindar (and specifically against Maeglin and Eöl).
The difficulty with the first theory is that the tone of the narration isn’t anti-Fëanor; in fact it’s very balanced. His story, and that of his sons, is described in the style of a tragedy, not a denunciation. The entire tone of it, culminating in the eulogy, “the mightiest of the Noldor, of whose deeds came both their greatest renown and their most grievous woe,” has a sense of tragically wasted potential for good, not of hostility towards the character. Even the Second and Third Kinslayings are written about in a tone more of tragedy than of pure condemnation - for such was the sorrow and confusion in the hearts of the Eldar in those days - and events that would have been known only to Fëanorians, such as Maedhros searching for Eluréd and Elurín, are included, though they could easily have been omitted by an actively hostile narrator. To the extent that the Fëanorians come across negatively, it is because they did terrible things, waging wars (well, battles) of aggression against other elves and against civilians and betraying their kin and friends.
There is a distinct difference between the tone the narrative takes of the Fëanorians and the tone it takes on Maeglin, with the former being more What tragically wasted potential and the latter being more just one of Fuck that guy. But the problem with attributing the account of Maeglin’s actions to narrative bias is that such doesn’t line up at all with the portrait of the Sindar generally. The Sindar are a major part of the two most complete Great Stories within The Silmarillion, the Lay of Leithian (Beren and Lúthien) and the Narn i Hîn Húrin (Tale of the Children of Húrin), and are portrayed positively in both - indeed, Lúthien is probably the single most unambiguously and impressively heroic character in the entire Silmarillion. (Thanks to the genuinely biased real-life storyteller, who loved his wife very much.) In the Narn i Hîn Húrin, Beleg and Thingol and Melian are all portrayed positively. So there’s no hostility to the Sindar here (though there is some cultural chauvinism).
Thus, to claim Maeglin was slandered, you have to theorize that the narrator had a very specific vendetta against only Eöl and Maeglin, and no one else, rather than them both being shown in negative terms because, like the Fëanorians, they did objectively terrible things. And the narrative includes statements (such as saying that Maeglin wasn’t a coward) that it would have no need to do if this was just a hit job. It acknowledges that elves taken captive by Morgoth can be controlled by him. It shows sympathy or pity for traitors under duress such as Gorlim (‘the Unhappy’) in the Leithian. And it specifically distinguishes Maeglin from this, as someone who chose to collaborate with Morgoth for evil ends. Maeglin’s actions in trying to take Idril captive at the sack of Gondolin are also actions in support of what Maeglin wants, not just Morgoth’s goals. Is he influenced? Yes; both Morgoth and Sauron show skill at corrupting others to evil ends. Is he controlled and brainwashed, and not to blame for his actions? No.
(There are other elements in places like Unfinished Tales that also support that this was Tolkien’s view of Maeglin. For example, in ‘Of Tuor and his Coming to Gondolin’, Maeglin is described as having made the Seventh Gate of Gondolin, the Gate of Steel, after the Nirnaeth. No negative or positive value is attached to this by the narration, but the symbolism is very clear and deliberate on Tolkien’s part. The first three gates - Wood, Stone, and Bronze - are geometric. The next three - Writhen Iron, Silver, and Gold - have symbols of the Valar: Thorondor, Telperion, and Laurelin. And the Seventh, specifically built late in Gondolin’s history, has Turgon’s crown, and unlike the others, does not have a gate: symbolically, putting Turgon above the Valar and indicating both his growing pride and shift to absolute isolationism, with Maeglin’s construction of it presenting him symbolically as an influence in both those things.)
Now sure, you can break things up and say “everything positive written about X character is true, and all the negative stuff is calumny by their detractors!” but that feels rather self-serving; and if you’re making assumptions like that, it elimates the point of discussing or analyzing the text as all, since anything can be claimed and no evidence is needed. At that point, you might as well write your own original story with the characters you want, rather than pretending they’re Tolkien’s.
Personally, I take the narration of the Silmarillion as being broadly omniscient when it’s convenient to Tolkien (characters’ thoughts are recorded even when they are unlikely to have confided them) and vague when he prefers to leave certain elements undefined (such as the fate of Eluréd and Elurín, though it’s possible that Tolkien just didn’t want to say outright “yes, these young children froze to death.” It was midwinter - they would have certainly have frozen before they starved, absent other rescue.)
In addition to the textual support for this view, I find it more personally interesting to play off of. I’m not generally drawn to rewrites or reinterpretations of stories that go with ‘actually the heroes did a bunch of bad stuff offscreen - and the villain did a bunch of good stuff that wasn’t recorded - and the villain had good reasons for all the evil stuff they did on-screen - so the villain is really the hero!’ (It’s why I didn’t watch Maleficent and am not especially interested in seeing Wicked.) And especially for The Silmarillion, where it’s theoretically possible for any of the Elven characters to return to life, I find it far more interesting to ask the question: “Starting from the basis that this character did do all the things depicted in The Silmarillion, could they come back (or be brought back) from that? What would redemption involve? What would it look like?” I’ve imagined redemption narratives for Maeglin, and for Maedhros and Maglor, starting from that premise (I tried one for Celegorm, but it doesn’t work, even in my head), and I far prefer fully grappling with and adressing the wrong they’ve done over attempting to rewrite, ignore, minimize, or justify it.
