i don’t need such an infallible authority to justify myself over others who hold to error
The purpose of the gift is not to justify oneself over others, either. Perhaps you have never been embroiled in a heresy like Gnostisism or Arianism, which took over as much as 80% of Christendom. It has certainly been necessary, throughout the history of the Church, to have an infallible authority. This does not just function in the person who is the successor of Peter, but also the councils. Most Christians accept the products of the first 7 councils as infallible teaching (like the Trinity, hypostatic union, dyotheletism, Theotokos, etc.).
God has presented His Word since the beginning, and seems like He did not need an infallible office.
Yes we will agree that he has protected His Word from the beginning. He established several avenues of expressing it, such as the prophets. He commissioned Moses to write the law, and stated that the “seat of Moses” was in the hands of the Sanhedrin. He enjoined the Jews to follow what they said, not what they do, demonstrating that teaching can be infallibly preserved even if men go astray.
Office(s) , magisterium, yes. Infallible ?
Not everything done by the Magesterium is infallible, either. There are very strict conditions.
I would think the Bride, the church , will be delivered pure and right on time, as promised and taught also (and not without challenges), at His second coming.
Indeed, Maranatha! But He prayed that we would have unity on earth before that time, and left instruction as to how this was to occur.
His Word (teaching) is protected in OT as in NT, the OT as prototype.
Indeed Jesus affirmed that the Septuagint is the Word of God. He has vouchsafed the Holy Scriptures, despite the efforts of the Pagan Roman Empire to eliminate them.
The problem lies in the interpretation of those Holy Writings. The Holy Spirit does not lead one person in one direction, and another in an opposite direction. One of the most important duties of the infallible authority is to provide a framework for rightly understanding the Holy Scripture.