What I have seen from textual studies in college, the manuscripts available today don't vary significantly. We have A & B readings for the great majority of the NT reaching back to the early second century. That the "Fathers" massaged them doesn't really concern me since we are after what the writers themselves intended. The Fathers were all over the map on all kinds of things.changes in meaning in the first 300 years. Kosta has made much more detailed study of some of these changes which have caused distress to some. However, my reply is that I believe that the Holy Spirit has led the Church to make these changes. Led, by the way, not frogmarched. Are Scriptures perfect? No. I believe, though, that the Church interpretation of them is what Christ intended.
As far as "choosing" what was in the so-called "Canon", read the rest of the 75 or so competitors and notice that there is no comparison. Scripture is as bright as day, the others being abberant and odd (eg. Jesus killing some boys who hurt his bird?). You and I would have spotted the real McCoys right along with those everyday believers who exchanged these letters for decades before the "Fathers" even got their hands on them.
Could we? I wonder. The protoevangelium of James and the Shepherd of Hermas were odds-on favorites to make the cut, and Revelation was pretty darn shaky - the East is still iffy, although they accept it as Scripture.
But to put things in perspective, NT Scripture did not occur before the AD 50's to 90's. The later books were written 50 to 100 years after that. And the revisions did not wane until the 300s. Scripture and Tradition went hand in hand, although Sacred Tradition chose Sacred Scripture. Not vice versa.
They don't because copies were purposefully "harmonized" with the oldest extant copies, so there is a great deal of "concordance" with what is genera ll (doctrinally) believed to be true. But this is retro-vision. We still don't know what the first century originals looked like. The oldest codex (circa end of the 2nd century) is, first of all, incomplete, and is not necessarily in agreement with other manuscripts, mostly fragments. For instance, you have long and short version of Luke's Gospel. Which one is is your pick?
There are numerous differences among manuscripts if not in artificially harmonized modern Bibles.
With all due respect, your Textual Criticism regarding the authorship of portions of the NT is the liberal Catholic version of reality. Having researched the matter myself, I don’t subscribe to this perspective.
Thus, the Holy Spirit’s work is more related to helping those called to understand what is already written, not creating new perspectives (traditions, interpretations) or correcting “errant” Scriptures. That which has been recorded provides sufficient information to lead believers into the correct apostolic teaching, given the Holy Spirit’s guidance, so my attention is on what the writers intended.