Christian or Jewish influence on the Quran is one of the oldest topics of inquiry Quranic studies. More recently, a scholar named Christoph Luxenberg (among others) has suggested that based on supposed Syriac readings of the text that the Quran may have begun as a Christian lectionary.

While the need to examine possibly Syriac etymologies in the Quran is widely accepted, Luxenberg's overall thesis is not, and his views are regarded as fringe.

As to "Quran manuscripts dating from the 5th and 6th centuries" that would be the biggest news in the history of the field. Intact Quran manuscripts before Muhammad? If he's found such manuscripts, then that would be a revolutionary discovery.

(Hint: They don't exist.)