The Old Testament was mostly written in and after captivity except for Dauet.(which was written in the time period of KINGS I and II) and some prophets writtings. The dead sea scrolls were not identical to the current OT, many variations but one have to admit in most cases the meaning is preserved. The jews had their canon setup right before the second destruction of Jerusalem, nothing has really changed after they scattered. However, since the dead sea scrolls are not identical then they are not the word of God because it can't be changed. The dead sea scrolls only proved the variations that existed when writing the jewish canon, different people and different authors. Since alot of errors and conradictions exist in the OT it is only a testament that most of those writings were a human act not inspired by God.
There is a book I would recommend for you to read:
God's scribes : How the Bible became the Bible
Author: Charles Isbell
He's a jewish scholar, I am sure you will learn alot from his book, it's just awesome.
The Quran says that those people wrote most of those books and called it the word of God which I believe is true. And actually, if you want to talk about the concept of God, there is really no difference at all between the God of the OT and the God of the Quran, only christianity deviated from it and attributed human characters to God (Son which is God can't speak for himself which is also a sign of imperfection in God).
I hope it is clear to you that we believe that not everything in the OT is corrupted, but we do believe that the NT is not the word of God at all. We believe that the INJIL(Gospel) was a book given to Jesus. The gospels totally depended on account witnesses (this explains why they never agreed on so many events and what Jesus really said ). Besides, the NT was chosen out of 24,000 documents around 300 AD.
Now proving Muhammad was mentioned in the OT is a different subject here, if you want to discuss it I will be more than happy to.