Septuagint "books"? Complete "books"? References please.
There are enough 2nd century NT manuscripts quoting from the Old Testament that differ significantly from the Hebrew (Masoretic) text. Whether these quotes represent Christian corruption of the Old Testament or a canon of an alternate version of the Jewish scripture is debatable.
I am not qualified to judge.
The existence of Essene's Qumran documents indicate that the Palestinian (Pharisaical) version of the Old Testament was not the only one, so the existence of alternative versions is not without foundation.
Once again - I am not qualified to comment.
On the other hand, New Testament books such as the Book of Hebrews (for example chapter 8 comes to mind) shows that Christian authors did not shrink from adding their own text to Old Testament verses in order to "harmonize" them with the emerging new theology.
Yup.
Both Josephus and the Talmud acknowledge that the Torah was translated by the Jewish scholars into Greek in the 3rd century BC. Jewish Encyclopedia further writes:
No serious source will doubt or expressly deny the existence of the Septuagint before Christ.