It’s interesting that Matthew quotes Jesus as saying that not a ἰῶτα will pass away/fall away/disappear from the law. That’s a Greek letter. Could this mean Matthew/Jesus are referring to the Septuagint translation of the Torah, specifically? Or at least had the Greek translation in mind, alongside the Hebrew Torah?
More questions, maybe unanswerable: Was Jesus speaking Aramaic here? Or Greek? Or Aramaic and then said ἰῶτα in Greek?
Here’s John Nolland, from his NIGTC commentary:
“To what does Matthew intend ἰῶτα to refer? While ἰῶτα is the simplest of the Greek letters (a vertical line), it does not make a particularly striking image for a tiny detail of the wording of the Law. The synagogue practice of giving the reading from the Law in Hebrew, followed by translation, may suggest that Matthew has the Hebrew text in mind. In that case ἰῶτα could represent yod (as frequently claimed), the smallest of the Hebrew consonants, and one which sometimes contributes nothing to the meaning.”
I find this less than compelling. If Matthew had the Hebrew Law in mind, couldn’t he have put a Greek transliteration of yod (or some other Hebrew letter) on Jesus’s lips?
Or is Nolland right, and Matthew simply translated Jesus’s “yod” into Greek, much as he would already be translating Jesus’s Aramaic speech into Greek (assuming Jesus did, in fact, primarily speak Aramaic)?
The larger interpretive question of what Jesus means theologically doesn’t seem to hinge on these language-specific questions, but I find them interesting all the same.
Words on the Word 
Interesting stuff!
I’ve wondered whether Jesus is pointing to himself and particularly somehow to the yod that is implied in his name:
http://speakeristic.blogspot.com/2009/12/prostitute.html
I had never thought about 5:18 as self-referential per se, but the possibility you raise adds even more to ponder!