My teacher (memory eternal) was an ultra-orthodox Jewish rabbi who converted to Messianic Judaism. He was a great teacher, not like most modern day ones that either demand specific interpretations and pronunciations, or are ignorant to history (like some Hebrew teachers who question the existence of Aramaic because its existence breaks Talmudic oral law). He was very knowledgeable in how ancient people lived and practiced Judaism and in gematria (which can be very frustrating, thankfully I was taught that early on).
the rest comes from a mix of some minor study with archaeologists (who usually know very little Hebrew but have some better understanding of how ancients lived), Jeff Benner (who made the video I linked you to), TheTabernacleMan, and (ironically) hebrew4christians
also, as far as the LXX and the copying error goes, I had no idea it was in the LXX. If it is, I don't believe it was an error, the LXX is (from what I was told) supposed to be the "end-all-be-all" when it comes to OT for Orthodox Christians. I'll study the verse a little more to see if I can find anything that it might mean, do you know if the Church has a specific interpretation or not?
Your background and training in it sounds neat! I think that the Messianic movement does have alot to offer. For example, its study of Hebrew brings up things that we might not consider, like the gematria you mentioned. That of course doesn't mean Orthodox wouldn't figure it out, but I think the Messianics' focus brings it out more, practically speaking. And of course the Messianics can be wrong about things too.
On one hand, I think the Jewish roots of Christianity is something important, and it is helpful to see how the rituals were. But not only does the Messianic movement tend to separate itself from non-Messianic Christians despite Christians' goal for unity, but I have become more doubtful of their idea that Jesus and the early Jewish Christians were so "Torah-observant" after all, even before it got to Paul. Two of the things that stick in my mind are Peter's vision that eating any animal is not "unclean" and Jesus' instructions to the healed man to carry his pallet on the Sabbath (not just that he healed on the Sabbath). I saw a list of such "breakings of the law", which was somewhat persuasive for me.Regarding the LXX,
you are right that the Orthodox rely on this text as it was passed down in the Christian communities. Yet I am doubtful that Orthodox must believe each word of the LXX in Greek is "infallible" or perfect. My guess is that with Holy tradition in general we consider it inspired, but that doesn't mean each piece is exact, since there are disagreements among theologians. What's even more important for me, though, is that the New Testament sometimes quotes verses that match the Masoretic instead of the LXX.
Of course, you can say that they are "both" inspired. But then what about the 10 commandments, which were obviously written in Hebrew, and by God Himself? The original Hebrew there would seem pretty important to me! Meanwhile, the Russian Church considers the Church Slavic translation to be divinely guided. But there are differences where Russian theologians consider the Septuagint to be better and different from the Church Slavic translation. My guess is that the Septuagint could therefore be considered in Orthodoxy similarly: the translators were divinely guided, but it might not be that each word is exact to what the prophet was inspired to write.
So LXX: inspired? Yes, in a way. As with other church traditions like icons.
"Infallibly" Exact? Not necessarily.