I quote the interlocutor; my reponses then follow.
“So, most Protestants, for example, accept as the authoritative word of God the canon of the MT, the 39 books of the Christian Old Testament, and do not include the deutero-canonical books.”
There is nothing about the Septuagintal tradition that compels the canon to include the deuterocanonicals. It can be Septuagintal without the deuterocanonicals just as it can be with Theodotion Daniel or the earlier alternative.
“But the Jews then were very familiar with the literature, language, and text traditions in a way that is difficult to replicate.”
Doesn’t it also matter (to you) that the Masoretes would have had anti-Christian bias in their tradition, or at least in their religion?
"The relation of the LXX to the deuterocanonical books is due to the fact that the great 4th and 5th century manuscripts (Vaticanus, Sinaiticus, Alexandrinus) included books not in the Hebrew canon."
I know, but those manuscripts were not representative of the (entire) canon. Only the Pentateuch is certainly from the Ptolemaic era, but we can (and do) speak of the "Septuagint" in reference to this time. (Of course, the 72 elders only did Moses.) The Dead Sea Scrolls had Greek-language scrolls that are technically "Septuagint", but those groups' canonical lists are rightly ignored in that case. Here, the identity of the books in the canon ("Twelve Prophets the Lesser," for instance) is rightly treated as different from the text of the book, which are also together different from the canon in which they find themselves.
"Jerome’s Vulgate originally did not include the deutero-canonicals, because they were not found in the Hebrew Scriptures; later he translated them."
In reality, he never actually changed his mind about the deutero-canonicals. But you see, if he were to include them, they have to come from the LXX, and so they do not have a place in an MT-based canon. However, this highlights that "canon" and "manuscript tradition" are different things. In particular, you can (and, in the form of the LXX, did) have a Greek-language manuscript tradition that translated the Hebrew (read: MT) canon. So you had one canon (say, Moses) but two manuscript traditions: of Hebrew and of the 72. If memory serves, Jerome even left the Psalmody to come from the LXX (in the Vulgate), to this day.
"With the deuterocanonical books in both the Greek and Latin versions, the Roman and Orthodox churches held these books in high regard, some holding them as fully Scriture, others as a sort of second-class canon."
From what I know, at Trent the Roman Catholics dogmatised the canonical status of the deuterocanonicals. Nevertheless, there was, at Trent, much debate against these books' canonicity.
"The Masoretes did not make new decisions about canon and text ... but merely transmitted the canon and text that was very ancient, going back at least to about AD 100."
One hundred years into Christianity is too late to presume absence of anti-Christian bias, and especially since the Council of Jamnia was explicitly counter-Christian. Moreover, AD100 is around when Justin Martyr complains that the Jews have recently changed the codices.
"So, any Masoretic biases would not seem to play a part in the establishment of the canon and text of the OT."
We would not know that if we do not compare the Masoretes to anything else. But when we do, there are differences, even with other (older) Hebrew-language manuscripts. They should not win simply because they were good enough for Jerome.
OT scholarship has a lot less criticism than NT scholarship.