I think the Council of Nicea was about establishing Trinitarian doctrine, not picking and choosing the NT canon?
I used to think there was massive editing, corruption, and modification of scripture.
But the Dead Sea Scrolls and the early manuscript evidence discovered in the 20th century generally point to the fact that these ancient religious people really genuinely tried to preserve what was originally written, for the most part. There is generally enough manuscript evidence now that scholars can tell what, if anything, was changed.
The fact that so much apocryphal literature was excluded from the canon shows that the various ecumenical councils genuinely went to the effort to identify forgeries, fakes, and later legendary accounts. But supposedly, three of Paul's epistles did sneak into the canon because modern scholars typically think those three were not actually written or dictated by him.