The foundation of their faith was the apostolic teaching. That's what writers of that era witness to - St Irenaeus most clearly perhaps. The scriptures are icons of that teaching, they witness to the teaching. That teaching didn't come in a vacuum - there was fertile ground in second temple Judaism, that's why St Paul went to the synagogues first. But many early Christians were from completely pagan backgrounds. They held to the faith they were taught, and that faith included scriptures. It certainly wasn't limited to them - it also included hymns, rituals, and teaching. That's what the NT witnesses - "they continued steadfastly in the teaching of the apostles, and in the Communion in the breaking of the bread, and in the prayers" or in another place "stand firm and hold fast to the traditions that you were taught, whether by word, or by letter from us."
You're busy arguing against some vague church council telling them what is appropriate - I'm not making that argument. Each church, with its bishop, is the Church in entirety and without deficiency. These men were entrusted with the public teaching of the Church, the teaching of the Apostles, to guard and maintain. "The things that you have heard from me among many witnesses, entrust these to faithful men who will be competent to teach others also."
Where they turned to for guidance was not only the scriptures, but also the apostolic teaching. They aren't in opposition, one contains the other. Just as the gospels are the crown jewel of the scriptures, the scriptures are the crown jewel of the apostolic teaching. "Lex orandi lex credendi" is real - the prayers and hymns and rituals of the church are every bit as much a witness to the apostolic teaching and continuity of faith as the scriptures.
If you want to talk about authority granted in scriptures, the NT itself doesn't tell the laity to submit to the scriptures, but to their elders. And to the teaching of the apostles explicitly outside of written instruction. And it also clearly outlines clerical roles and gives teaching for ecclesiastical structure and discipline but is silent on the question of the canon.
The ultimate authority rests in Christ. He alone is our Pastor, Teacher, and Priest. His Spirit animates and makes alive the Body which is the Church, and He is active in that role every day, at all times. Every liturgy we confess "Christ is in our midst, He is and ever shall be."
The scriptures are an icon of Christ, but they are not supreme. And, as the Church itself is the Body of Christ with Christ as the Head, there is nothing over it. How could we subordinate Christ to scriptures which teach of Him? Doesn't He say "You diligently search the Scriptures because you think to have eternal life in them, and these are they bearing witness concerning Me, yet you are not willing to come to Me, that you may have life." I'm not pointing those words to you, but showing that such a thing is absolutely possible to do.