be that as it may, once again we return to my point: the muslim cannot base their argument on the full text and as such they must provide some reason as to why they cannot and for why the christian can.
i noticed that you acknowledged that the muslim deity had told the christians at the time of muhammad to look at the gospel which was in their hands. this is actually a point against islam because we certainly have manuscripts dating from before the time of muhammad and after. we certainly know what gospels these christians would have possessed and they are identical to the message you will find in our current bibles.
that said, islamic history is quite clear that the qur'an has not been passed down perfectly:
1. ibn masud rejected the qur'an which zaid ibn thabit compiled and called it a forgery seeing as he maintained that the qur'an should have only 111 chapters instead of the current 114. ( The people have been guilty of deceit in the reading of the Qur’an. I like it better to read according to the recitation of him [i.e. Muhammad] whom I love more than that of Zayd Ibn Thabit.)--- Ibn Sa’d, Kitab al-Tabaqat al-Kabir, Vol. 2, p. 444.
2. ubayy ibn ka’b who was the best reciter believed that zaid's qur'an was missing two chapters and so those who believed in zaid's qur'an were forced to ignore some verses which ubayy used to recite (Umar said, “Ubayy was the best of us in the recitation (of the Qur’an), yet we leave some of what he recites.” Ubayy says, “I have taken it from the mouth of Allah’s Messenger and will not leave it for anything whatever.”) --- Sahih al-Bukhari 5005
3. ibn umar was adamant that teh full qur'an had disappeared (“Let none of you say, ‘I have learned the whole of the Koran,’ for how does he know what the whole of it is, when much of it has disappeared? Let him rather say, ‘I have learned what is extant thereof.’”) --- Abu Ubaid, Kitab Fada’il-al-Qur’an
4. aisha, the wife of muhammad is reported as claiming that about 2/3 of surah 33 is missing (A’isha . . . said, “Surat al-Ahzab used to be recited in the time of the Prophet with two hundred verses, but when Uthman wrote out the codices he was unable to procure more of it than there is in it today) --- Abu Ubaid, Kitab Fada’il-al-Qur’an
5. aisha's goat is reported as having eaten some parts of the qur'an and this is why they are missing from the modern-day qur'an (It was narrated that Aishah said: “The Verse of stoning and of breastfeeding an adult ten times was revealed, and the paper was with me under my pillow. When the Messenger of Allah died, we were preoccupied with his death, and a tame sheep came in and ate it.”) --- Sunan Ibn Majah 1944
First, Surah 33:6 declares that “The Prophet is closer to the Believers than their own selves, and his wives are their mothers.” However, Ubayy ibn Ka’b and other early Muslims held that a phrase (“and he is a father of them”) is missing from this verse. Even the great translator Yusuf Ali admits this in his commentary. Ali writes: “In some Qira’ahs, like that of Ubayy ibn Ka’ab, occur also the words ‘and he is a father of them,’ which imply his spiritual relationship and connection with the words ‘and his wives are their mothers.’” --- Abdullah Yusuf Ali, The Meaning of the Holy Qur’an, Note 3674
now you can go ahead and claim that all the above is inauthentic and that is perfectly fine yet the very fact that there is recorded disagreement on the preservation of the qur'an by muslims themselves proves my argument. in the same way that the muslim will say that we ought only to believe in the "authentic hadiths" so the christian maintains that one ought only to believe in the books which are part of the biblical canon. there is no argument for why we should reject what i have shown above that would not equally apply to the christian position for why we ought to reject all books outside of the canon. so no, islamic history tells me that the qur'an has not been perfectly preserved.
