If I may, I became a Catholic after leaving the Jehovah's Witnesses and helped to teach RCIA classes (the Rite of Christian Initiation for Adults which introduces and welcomes new adult Catholics into the Church).
From my teaching materials (which include several ecumenical volumes from Oxford):
"The oldest Bible is the Septuagint."
The Septuagint is a Greek translation of the Hebrew Scriptures. This is not the oldest Bible collection. As a translation, the Septuagint is based on something older, the Hebrew texts themselves which make up the original collection. Older collections are found in the Qumran/Dead Sea scrolls. However, these do include the Deuterocanonical books found in the Catholic canon.
"Jesus used the Septuagint."
This doesn't seem likely. Jesus spoke Aramaic and Hebrew. The texts read in the synagogues in Second Temple Israel were composed in Hebrew (some may have been in Aramaic), and regardless of the common tongue Jews speak in for everyday conversation, historically they have been bilingual, teaching Hebrew to their children in order to read and understand the Scripture readings and prayers used in worship. The Septuagint was translated in Greek for the Diaspora, the Jews who lived and worshipped outside of Israel.
"The writers of the Christian Greek Scriptures quoted the Septuagint."
While the Watchtower has repeatedly taught that the New Testament writers quoted from the Septuagint, Biblical philologists have demonstrated that this is unlikely, at least as a general rule. There are too many variations in the quotes used, and often it appears that the writers were often employing midrash (a Jewish teaching technique of quoting the text in specifically unique ways in order to make their arguments work out in a certain fashion). The early Church Fathers, however, did begin to use the Septuagint as their official text.
"The Septuagint was the original canon of the Old Testament."
Interestingly, Catholics and Orthodox Christians will tell you "no." Even though they accept the so-called "extra" books found therein, the idea of an official "canon" was invented by a heretic bishop named Marcion of Sinope. Influenced by Gnostic beliefs that written texts should be the basis for truth instead of theophanies or epiphanies of God, Marcion made a canon which excluded all Hebrew Books and accepted only select letters of Paul plus an oddly reduced version of Luke. In response, the Church Fathers began to develop an official canon. It would take more than 200 years, with the New Testament canon being approved in the 4th century and the Old Testament being approved at the Council of Trent about 1000 years later. Being that "canons" are a Christian invention, there has never been an "officially finalized" Jewish canon even though today Jews speak of their Tanach as a canon.
Because the Watchtower demands like Marcion that the ultimate form of revelation from God must be the written Bible, they demand the books to have been written as if this was the original plan of its authors and the communities that composed them. The Witness view can never match the historical reality of how the Bible developed and why.