There was no OT canon until the second century BC, giving great opportunity for Jewish scribes to amend the writings. How do we know our received OT is inspired?
First of all, I am not sure where you are getting your information that there was no canon before about AD 160 or so. Josephus tells us that it was Ezra, in the 5th century BC, who put the canon together in his Annals of the Jews. Exactly how true the account of Josephus is, it is hard to prove. It is possible that Josephus overstates, but it is not likely that he is completely wrong. Daniel tells us in the sixth century BC that he was reading from Jeremiah, calling it “Scripture.” The person giving you this late date is probably a theological liberal or possibly even an unbeliever, so you should take this with a grain of salt.
The person you are quoting from implies that priests were amending the Scripture at their will in the second century BC. The problem with this is that we have copies of the Septuagint, which was a translation made in the third century BC. We also have the Samaritan Pentateuch–a translation made in the sixth century BC. This statement is either wholly wrong or, more cautiously, is a gross overstatement of how much amending could have happened as late as the third or fourth centuries. We even have Dead Sea Scroll manuscripts from the third century BC. The statement about alterations in the second century do not stand up to the evidence. The miniscule change in the Hebrew Scripture between the time of the Dead Sea Scrolls and the Masoretic Text (about 900 AD) is strong evidence that the Jews were very reticent to amend their holy Scripture.
It is true that there were quite a few non-canonical books written in the third and second centuries BC. All of these were written after ALL of the canonical Old Testament books. Many of them were in Greek, and NONE of them were ever accepted by the Jews as canonical. Not one. Therefore, although it is true that the Jews wrote a lot of books (so do modern people), this has little or no effect on how we ought to view our received Hebrew scriptures. Again, you are reading people who are giving you a quite distorted view of the data. Most of what they say is true, but definitely not all, and the facts they are telling you are being grossly abused to make a point which is simply not true.
I agree that there is no proof that those who collected the Hebrew canon were infallible. This statement you made is largely true. What we have is the final result. The greatest evidence for the inspiration of the 39 books of the Jewish Bible is the quality of the books themselves. That Genesis, Exodus, Leviticus, Numbers and Deuteronomy are inspired is self-evident from the books themselves. The literally hundreds of messianic and other predictive prophecies in the Old Testament are wonderfully strong evidence for their inspiration. Just read Isaiah and then read Tobit (from the Apocrypha), and you will know right away which is inspired. Consider the miraculous nature of the prophecies in Isaiah 53 or Isaiah 11:1 or Isaiah 7:14 and 9:1-6. What about the statement in Psalm 22 that the Messiah would be pierced in hand and feet? What about Micah 5:2 where we learn that the Messiah will be born in Bethlehem, or Zechariah 11:12-13 in which the price at which Judas betrayed Jesus is given, hundreds of years before the events. As Peter noted in 2 Peter 1:16-21, “We have the prophetic word of the Jewish Old Testament made more certain. This is true because Peter saw the prophecies fulfilled before his own eyes. We have great reason (and I am only scratching the surface) to believe that “All Scripture is inspired by God.” (2 Tim 3:16). What distinguishes inspired from uninspired books is the quality of the material. This provides such a strong case for inspiration that the books we have less “proof” for being inspired, such as Esther, are, by implication, also inspired. Lots more evidence for inspiration of the Old Testament is found in my book Reasons For Belief (www.ipibooks.com).
By the way, Daniel definitely is NOT a forged book. Please get a copy of my book, Daniel, Prophet to the Nations (available at www.ipibooks.com) for a boat-load of evidence to undermine this spurious theory.
Like I say, the books speak for themselves, as they have for nearly three thousand years. We do not have to have full access to the precise means by which they were collected in order to know this from the quality of the writings themselves.