How can the authenticity of the Holy Quran be proven, and that it has not been distorted by adding scientific facts, despite the existence of verses like: "Indeed, it is We who sent down the Dhikr (i.e., the Quran) and indeed, We will be its guardian"?

A distinction must be made between "theories" and "facts" concerning scientific miracles (i'jaz). The Qur’an is a book of guidance, and its primary miracle is linguistic and rhetorical. When considering what some contemporary scholars say about aspects of scientific miracles, one should not assert definitively that scientific theories are indisputable facts, or that what they say is the interpretation of a Qur’anic verse. The meaning must conform to the Arabic language, and the speaker must be a scholar of Islamic law. The meaning must not contradict any verse, sound hadith, or scholarly consensus, and one should avoid forced interpretations. As for the claim that these things could have been inserted into the Qur’an, this possibility is rejected even by the enemies of religion, because the Qur’an is preserved by Allah's protection and transmitted through continuous, widespread narration (mutawatir).