Just skimmed this page only. Here are some links and excerpts that might clear up a bit of the confusion regarding the Islamic belief regarding other Scriptures and/or abrogation:
"Being the final, all-encompassing and preserved against any change, the Islamic Law abrogated all the previous Scriptures which sustained human interpolations. Abrogation generally takes place with regards to secondary matters of law. As for fundamental principles, like the Oneness of Allaah, The Most Exalted, the prohibition of idolatry, and the fundamentals of worship – matters that form the basic call of all the Messengers may Allaah exalt their mention – they are not subject to abrogation."
Dominance of the Quran over previous Scriptures - Islam web - English
Evidence That Islam Teaches That There Was Textual Corruption of The Christian and Jewish Scriptures
"Imam az-Zamakhshari clarifies... The Qur'an claims to be a confirmation, protector and touchstone of the Truth contained in the Torah and Gospel. (Az-Zamakhshari, op. cit., vol.2, p.575, cited
here)
Ibn Kathir also reaffirms this position...
(but a confirmation of that which was before it) in reference to the previously revealed Divine Books, by which this Qur'an testifies to the true parts that remain in them and denies and refutes the forged parts that were added, changed and falsified by people. The Qur'an accepts or abrogates whatever Allah wills of these Books, (Tafsir Ibn Kathir,
Source)
Ibn Jarir said, "The Qur'an is trustworthy over the Books that preceded it. Therefore, whatever in these previous Books conforms to the Qur'an is true, and whatever disagrees with the Qur'an is false.'' (Tafsir Ibn Kathir,
Source, You can also see Al Tabari saying this himself in his commentary on Surah 2:41)
Imam al-Tabari relates from the eminent Jurist Ibn Juraij (80-150 AH/699-767 CE) that if the people of the book quote something from their Bible that disagrees with the Qur'an then we reject it but if it agrees with the Qur'an then we accept it. (Source)
Imam al-Bahgawi also quotes the opinion of Ibn Juraij in his commentary. (Tafsir Al Baghawi, 1/65) Abu al-Layth al-Samarqandi (d. 373 A.H.), the well known Hanafi scholar in his commentary of the Qur'an known as Bahr al-'Ulum on Surah 5, verse 48 says that the Qur'an acts as a judge over the previous scriptures. This indicates that the Qur'an judges whether something is true or false. (
Source)
The Qur'an is the protector of the Gospel and Torah. But which Gospel and Torah? It is the original revelations sent to Jesus and Moses (peace be upon them both) and whatever of it survives today (e.g. prophecies of Muhammad peace be upon him to come) in the scriptures of the Christians and Jews today."
Refuting The Argument Regarding The Qur'an Being A Confirmation Of The Bible