One other thing we need to take into consideration, is that Jesus was speaking by an large to the Jews. When Jesus said "what saith the Scriptures" what would they have thought? The Hebrew Scriptures, of course.
Where were these Jews at? In Israel. Was Hebrew a lost language at that time so that they ONLY used the Septuagint? No. 80-85% of the documents that are found in the Dead Sea Scrolls were in some form of Hebrew (most of it biblical Hebrew). The scholars used it extensively. When standing in the synagogue, Hebrew was supposed to be the language of prayer and study.
Alfred Edersheim wrote:
If Greek was the language of the court and camp, and indeed must have been understood and spoken by most in the land, the language of the people, spoken also by Christ and His Apostles, was a dialect of the ancient Hebrew, the Western or Palestinian Aramaic. It seems strange, that this could ever have been doubted. A Jewish Messiah Who would urge His claim upon Israel in Greek, seems almost a contradiction in terms. We know, that the language of the Temple and the Synagogue was Hebrew, and that the addresses of the Rabbis had to be 'targumed' into the vernacular Aramaean—and can we believe that, in a Hebrew service, the Messiah could have risen to address the people in Greek, or that He would have argued with the Pharisees and Scribes in that tongue, especially remembering that its study was actually forbidden by the Rabbis? The Life and Times of Jesus the Messiah, Book II, Chap II, page 131.
It is without a doubt that the Septuagint was also widely used by the populace during the period of the early church. But was all of it considered Scripture - or, as Jerome seemed to deem it, was it a mixture of true Scripture and some profitable but not canonical reading? Philo quotes from the OT quite a bit, but never quotes from the Apocrypha. Not that Philo is authoritative regarding what IS Scripture. He is just there for evidence of what was considered Scripture at the time. Of course, this is an argument from silence and there are several OT books that dont' get quoted in the NT either and yet are Scriptural. However, I think it is odd that the TRANSLATION of the original with some added books has been deemed more authoritative than the Hebrew Old Testament (Masoretic or older fragments) and the word of the Hebrews as to what their Canon was. We know that the Jews have been blinded as far as Christ goes. But are we to expect they are blinded as to what their own Bible is as well?