You asked, “If the Bible is that instrument, then why didn’t Jesus himself write it, or dictate it to a scribe as Jermiah did ...” Are you seriously denying that the Holy Spirit inspired the writing we have as our New Testament? Is that a Catholic belief there?
No, I am denying that the book itself can serve as a substitute for the Church, the persons whom the Scripture referrs to as “the body of Christ,” and ourselves as his members. As to the Scriptures, the fact that only some Christian writings were included in the canon and were read in the assembles means that some human agency had to decide which writings were revealed truth and which ones were “inspired” in a lesser manner. The difference between Catholics and the Reformers was really only in deciding whose authority in the matter of Scripture was greater.