This is a fine point and one more Christians must consider. I totally beleive the bible is the inerrant word of God in the original Hebrew, although I would not discount the Greek since Jesus may have quoted from the Septuagint. One more point. You also must take in totality the all the scripture. I believe it's all an integrated message that points to Christ.
All of your points are well taken here.
Jesus was quoted by the Gospel writers from the Septuagint, but that doesn't mean He actually quoted from the Septuagint.