I've tried everything, but nothing seems to be able to debunk the claim that the Quran is unique, and cannot be imitated.
It is an irrelevant claim. Uniqueness does not indicate divinity in any way. Imitated is not a sign of divinity either. If I challenge you to make a pizza exactly like I do and you fail, as exact reproduction is impossible, does that mean my pizza is divine or I am as you can not copy my pizza exactly?
Stop to consider if the very idea being put forward is valid itself before worrying about debunking it. Nonsensical claims do not need to be refuted as the claim is a a non-starter.
The closest thing to debunking this which I can find is that the early Meccan chapters are actually Saj,
So part of it is not that unique in form. Ergo the previous point is partial moot.
and that only later Quranic chapters start to become unique. If this claim is true, the Quran can be imitated (and obviously has been since we have many great pieces of Saj in both Arabic and non Arabic), and it also shows that the Quran as a text evolved, with the author becoming more and more competent as he had more and more experience in producing literary masterpieces.
This is nothing new. Many forms of art have used existing methods while developing new methods. It is how new forms and methods of art are actually created. At some point someone was the first at something. Someone was the first to use stone in art. Someone was the first to use stone for representation art. Someone was the first to use stone to create statues of humans. Someone was the first in using previously unused material for statues rather than stone. Development is why we have more forms of poetry than simple rhyming poetry only. Being the first to development a new form and/or method is not a sign of divinity but of creativity.
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