When I mentioned about Mt 5:18, the main point is that even the least portion of the commandments should not be ignored, which was the main theme of the message there. However, from that statement we gain the glimpse of the language involved there as a secondary information, which means Hebrew Bible in that case. I don't deny the key point of the message in Mt 5:18 was the importance of every bit of the commandment.
Also, Mt 23:35 talks about Martyrs starting from Abel thru Zechariah, then we can have the glimpse of the order of the Bible too, because LXX doesn't explain why Jesus ended up with Zechariah, while Zechariah is mentioned as the last Martyr in the Masoretic Text.
Also, Jesus spoke to Paul in Hebrew and Paul was explaining hisn own testimony about his salvation and calling, but from there we notice that Jesus was speaking in Hebrew to Paul who was fluent in Greek as well.
Also, Luke 23:38 says Superscription was written in Greek, Thisn is King of the Jews, in Greek, Latin, Hebrew. The content is the most important, but we can get the secondary information that it was written in Hebrew for the sake of Hebrew speaking Jews.
So, we get sufficient evidences that Hebrew was used among the disciples, and that Jesus was mentioning about Hebrew Bible. I have not encountered any single evidence that Jesus and Disciples used Greek Bible-LXX instead of Hebrew Bible.