Another point I found compelling is this:
The little things the two schools of thought find to argue about says a LOT about what they do NOT argue about.
This means that the common critic's claim that Scriptures have been corrupted beyond recognition due some variant of the old "telephone game" of successive copies diverging is overwhelmingly denied by the lack of arguments about them.
Where are these corruptions?
I've noted many of these corruptions already, and I've demonstrated that, contrary to your contentions, they were material, substantive and influenced the evolving Christian doctrines. But, even so, it's likely that we've uncovered only a small portion of these "corruptions". Why? Becasue we know for a fact that they were sytematically and ruthlessly supressed starting during the time of Constantine.
Prior to Constantine's conversion, Rome had ruthlessly and systematically supressed Christianity, right? Every Christian remembers the times of the "martyrs". They have no problem appreciating the full extent of its horidness.
And yet, these same Christians naively believe that once Constantine converted, he embraced all forms of Christianity. He didn't. Rather, he called the Nicene Council to resolve the many and varied disputes within Christianity and to create a standard Christian doctrine for once and all. That doctrine was then enforced with the same ferocity, in fact more ferocity, over the subsequent centuries that Rome had originally directed toward Christians in general.
In other words, once the canon started to become fixed during the time of Constantine, "heretical" documents were systematically destroyed at worst or simply ignored (that is, not copied for posterity) at best. Versions of the cononical gospels that differences from those endorsed by Rome (such as, perhaps, the original Matthew which was written in Hebrew) and the original Mark (which contained only "sayings of the Lord" though "not in order") disappeared over the subsequent centuries.
Having said that, I would not be surprised if some of them are eventually found. Remember, it's only been within the last century that the Nag Hamadi Library and the Dead Sea Scrolls were both discovered, and they contained a great many "heretical" Christian/Jewish texts that had been supressed nearly out of existence. It would not be surprising for someone to one day stumble upon the original Matthew or the original Mark, or perhaps Luke or John or versions of the letters of Paul that differ markedly from our received ones. Given the extent of the suppression, the odds are not great, but with the passage of sufficient time, it's possible.
While I certainly don't condone the actions of the popes throughout history, there was nothing in principle wrong with gathering the leaders of Christianity together at multiple conferences over the centuries to address issues of controversy and weed out teachings deemed to be false by the majority. The alternative is to allow false teachings to proliferate without bound.
Acts 15 describes the first of these councils in Jerusalem with the actual apostles present which serves as a model and an implicit endorsement of the process.
I will concede that at each of these major events we are counting on the Holy Spirit to oversee the decisions that were made under the doctrine that inspired scripture must be preserved the same way if it is to reach us so that we can obey it as commanded. This is an article of faith backed up by the Scriptures that Jesus Himself endorsed.