Why would we need to assume anything about God's direct involvement in the authorial processes taking place within Matthew's mind as he decides what to write about and how to selectively articulate his gospel. It seems to me that while your imputation of what it would be to be reasonable can be appreciated as we assess the contents of Matthew's gospel, you're putting way too much stock in the extent to which you think we have to assume that God directly 'caused' the writing of the gospel itself. Moreover, in my estimation, I think we probably need to be realistic and assume that God Himself did not create the various systems of human language themselves, especially not those used in the writing of the Bible; no, "we" humans created those systems of language.
So, while I understand your grievance and we can all say we wish God would have made the bible easier to understand on the whole, I also think it is more than reasonable to assume that where we find human writing within the bible (i.e. everywhere in the bible?), then therein we can find at least some of the answers we're looking for by paying attention to, and studying, the thought patterns, literary intentions and language use of the human writer of each biblical book or letter. In this case, if the motif of "fulfillment" is important to Matthew, then we'll have to carefully study his entire gospel to 'see' what he means in his attempt to illustrate his reference to that motif.
Cutting to the chase: the point is, Matthew seems to think "fulfillment" of the O.T. by Jesus is, and can be, something different in nature than what you're seemingly thinking it should be in some totalizing fashion.