Important line: "And those books were decided on [by the famous synods and councils]because they were in widespread use and respected by everyone."
That's exactly right. It's what we Catholics call the "Sensus fidelium" ... the sense of the Faithful. In this case, the actual practice. The big-T Tradition.
Correct. I rode my bike past St. Jerome’s house every day for a year while studying in Rome. I would say to myself, Yup, that is where the Bible was assembled.
St. Jerome and his lion had two hundred or so fragments on his desk including fragments similar to the Muratori document. Myth? You wish. Give a high five to St. Augustine for encouraging St. Jerome to stop being slothful and start being industrious.