Dear Martyr:
Exactly, the prophetic word is "sure" or "reliable" precisely because it is "infallible." Concepts are explained differently from one culture to another, from one language to another, from one era to another. You won't find the word "infallibility" from the 15th century B.C. to the 1st century A.D. because that's just not how they spoke about these things. But, the concept is very much the same. The scriptures are "blameless," they are "without spot," they are "reliable," they are "sure," they are "perfect," just different ways to say that they are "infallible."
As for the Old Covenant, of course it was replaced by the New Covenant not because God made a mistake, but because it served its purpose in His plan of redemption and it was fulfilled in Christ. Whatever was lacking in the Old Covenant was due to the sinfulness of man, not due to the problems in the Law itself or errors in the scriptures. If man had not been sinful, the Law would have brought life. But, since men were sinful, the Law that was supposed to bring life brought death, which is why Jesus had to fulfill the Law on our behalf so that we might become the righteousness of God in Him.
As for the way that God uses those whom He has ordained in the church to take care of God's people and to pass down the traditions that come from the Lord Himself, you really need to consider the ministerial authority that is vested in the officers of the church (notice that I did not say magisterial authority), as it is communicated in passages such as 2 Th 2:15, Mt 18:17-20, 2 Tim 3:14-17.
You know, it's funny that we're having this conversation, because all of these concepts are explained in Michael Horton's book, "The Christian Faith." You would really benefit from reading that book. It explains all the fundamentals of the Christian Faith, from a historical perspective. It explains how these doctrines were developed, how they are grounded in the scriptures, and how they have been handed down and interpreted differently. Just imagine that you're having a conversation with Michael Horton as you read his book, just like you and I are having a conversation through writing and reading.
Just a quick question, do you know what I mean by distinguishing between the ministerial authority of church leaders and the magisterial authority of the scriptures?
(It's okay to say, "I had never heard of this distinction until you mentioned it, and then I googled it and learned something new today"... just like it's okay to read a book about theology, learn new concepts, and improve your understanding of the scriptures)
Blessings,
Renoncer