The Septuagint is not regarded by Jews as a valid representation of Scripture.
It's version of Daniel in particular was one of the reasons the LXX was rejected (and to this day the Septuagint holds no place of importance in Judaism). There are huge additions in the Septuagint version of Daniel which, when the formalizing of the Jewish canon occurred in which Daniel was eventually placed among others in the Ketuivm (Writings), these Greek additions were excised.
The idea that Daniel was a prophet is incompatible with our history for, as noted in my previous post, the historical Daniel is supposed have died many years before the Babylonians invaded. (Ezekiel 14.14, 20; 28.3) Daniel is an important historical figure to us Jews, and the conclusions you have come to, while popular with Fundamentalist Christianity, is at odds not only with Judaism, but Orthodoxy, Roman Catholicism, teachings of the Church of England, Methodists, Presbyterianism, and most others mainstream Christians for this very fact regarding Jewish history.
The Catholic Church, on the other hand, recognized the Alexandrine Septuagint and its contents as the Old Testament. Because the Book of Daniel appears in the Prophet section of the LXX, the Catholic Church has kept its placement where it appears in that Greek version, including the Greek additions found in the LXX (The Song of the Three Jews, which are additions to chapter 3, the story of Susanna and the Elders, which is marked as chapter 13, and the tale of Daniel, the priest of Bel and the Dragon, which makes up chapter 14).
When Protestants decided to reject the complete Alexandrine canon of the Hebrew Bible honored by Catholics, they did not immediately remove the texts from their Bibles. Instead they moved them and their additions to a section between the Old and New Testaments, marked "Apocrypha." They left the remaining books in the LXX order however, and some mistakenly made the claim they did this because they were following the approved Jewish canon (which has a different book order). Leaving them in the Catholic book order, many have mistakenly assumed Jews have viewed this book as prophetic since the Catholic order has the book among the prophets.
There was no "shift" of the book of Daniel to the Writings section around the 5th - 8th century C.E. "as confirmed by Koch" as there has never been formal codification of the books that follow the Hamesh Megillot. These "last books" following Megillot are Daniel, Ezra-Nehemiah, and Chronicles. They has never been a formal place or order for these last books, so they could never have been moved to a different section as you cite.
Also, the Qumran texts represent collections of Hebrew texts preserved by a Jewish sect that existed on the outskirts of Judaism. While the find of these texts is quite phenomenal on the philology side of Scripture study, they play little more significance as they also contain a lot works not recognized by mainstream Judaism about the End of Days and how those who originally owned the Qumran texts believed they were God's only true religion that would survive to see paradise restored on the earth.
Their texts, remarkable as they are, are also the equivalent of someone finding preserved copies of the New World Translation among a few JW publications some 4,000 years from now. It would be remarkable, yes, to find them at that point, but hardly representative of what the rest of Christianity really believes nevertheless. Thus pointing to a version of Scripture, the LXX, that Jews don't find authoritative and the Qumran scrolls to support conclusions that clash with Jewish history of its own figures and the development of its own works of religious literature is an odd way to prove a point.