Interesting how so many discussions touch on similar aspects. This is currently one of the issues in the Chapter by Chapter discussion.
I have been told

that my suggestion of some kind applicability of the Lilith myth to Shelob is a no-no: it brings outside "baggage" from the Middle East into Middle earth and destroys the illusion that the subcreated world is the extant one. I am supposed to hold this in abeyance and deny any similarity while I read. Yet for me, this application does not destroy my sense of Middle earth. It simply expands upon a context. Perhaps another way of explaining this for me is to say that the allusions are subtle enough and the character Shelob well formed enough in her own right that I don't drop out of Middle earth.
Yet just a few chapters back I did feel that the spell was gone, in "Journey to the Cross-Roads." At the time we discussed the chapter I said I was unconvinced by the descriptions of incarnate evil. And that, while I liked the graffiti on the fallen statue of the king, much of the symbolism seemed forced. It pointed, for me, too much to the effect Tolkien was striving for "consciously in the revision."
So, I am going to posit something here. It is not so much the semantic meaning of a situation or event that destroys the spell of created world: it is how that particular bit of world is written. Faramir may have jumped forth fully clothed from Tolkien's mind, but his depiction is written with fidelity to some hidden imperative in the story. "Journey to the Cross-Roads" was written after substantial parts of this Book were planned, when Tolkien realised he needed to add some extra time in to balance the two plotlines. Somehow, this kind of planning created 'overwriting' for me. It didn't for others.
How to account for that difference might be interesting. But for now my only way of understanding what breaks the enchantment is the ability of Tolkien's words to hold for me the meaning the story wants. That likely begs the question of what we as readers bring to the text.
Related to this might be the issue of why Tolkien holds no enchantment for some readers, even readers of science fiction and fantasy. They're gone from the start. Does it come down to a willingness to be enchanted? Heart's desire as a reading strategy?