There's a good case for morality as emerging from storytelling. Having a shared set of well-known stories, like the old testament, or the vedas, gives a shared context of reference.
Your picture of authorial authority isn't right. For instance, how can it deal with Tolkein contradicting himself, or changing his mind? His authority is from the coherence and power of his vision, and especially the detail of his world building. He began by developing different languages he found interesting, then framed the Silmarillion around the history needed for the languages to have evolved and changed. As far as I know no other author has attempted world building on such detail or scale. The cultural and moral parameters of the stories is clear, and a dynamic continuing world off-screen and before the stories is clearly compellingly implied.
The new Starwars films make many mistakes, like Luke's attitude to his lightsabre being out of character, multiple people surviving direct light sabre hits, and the Emperor coming back from the dead in a way that makes no sense, a lot of inexplicable force powers that seem contrived for the plot, zero mention of anything that happened in the prequel trilogy, and no picture of continuing events and history off screen. Whereas, the original trilogy drew heavily on The Hero With A Thousand Faces, and the films of Akira Kurosawa, for mythic depth, characterisation, and worldbuilding.
Homer wrote in a way that was very compelling. The Illiad can be taken as pure action, or it can be understood as commentary on the waste and sufferings of war. We find the same with Beowulf, also from the era of oral history, where it is subtly implied Beowulf himself has become monstrous.
Greek stories were told in play form at competitive Dionysia festivals. And we can see from how Medea is identified with her role in a specific play by Euripedes, rather than from older Jason and the Argonauts, how the stronger story could win not only laurel wreaths, but become canon.
The story of Noah is thought to have been based on The Epic Of Gilgamesh. But the details and especially the moral drawn, are substantially different. Getting accurate oral transmission required a dedicated class of people, and a cultural priority on recitations - the prominence of genealogies in the bible can be understood in this way, as bolstering the authority of those in power. There must have been especially compelling versions of the bible stories, which became canon, before writing made continuity much easier to assure, and ceased the changes and reinterpretations for new audiences.
It is interesting to note the power of specific events like the death of George Floyd, as being a shared reference, in a kind of cultural text, to be a touchstone that shapes an era. Who decides what will be canon? They say journalists write the first draught of history. A really great historian like Anthony Beevor is able to go back over the record, fill in gaps, add perspective, and reframe the understanding of events. I think Niall Ferguson said each generation must arrive at it's own understanding of it's relationship to it's history - that history isn't 'dead events', but a mine to draw lessons from to understand ourselves.
We often find simple game-theory dynamics behind the simplest elements of stories that illustrate morals, that promote unstable equilibria. To survive as memes, they must as well as providing some benefits for a society influenced by them, be compelling, memorable, and work told to different audiences. I would look to emergent canonicity as the result of competition in the memesphere. An analogy I like is the blockchain: multiple people might submit numbers they have mined, and inconsistencies are decided by which record is more widely adopted, then a '2nd draught' goes into a block. Changing these is harder and rarer, but does sometimes happen - less often the further back. Then you can get a hard fork, where the ledger splits. The larger part of the fork likely has more activity, more copying, hashing power, and tends to draw people into the network. But there's a lot of narrative around hard forks, that shapes what happens.
One way to define postmodernism, is the fragmentation of a shared body of narratives, especially those that required some measure of assent on pain of death - like Abrahamic faiths, and Ancient Greek religion. But from the above framing, we can understand we are really returning to a 'public ledger' system of how a community defines itself, with an emergent dynamic canon of the history it defines itself by, and how that is told.