The question is very relevant. If the scriptures are the word of God then God knows His word and has preserved it. Nothing that the councils could have done could have changed the final outcome because no book that was not the word of God could have made it to the canon. See the quandary?
And again, that does not necessitate Sola Scriptura, nor does it automatically invalidate the authority of the Councils or the theology of Sacred Tradition. I have still never received a good answer as to why the authority of the Councils magically stop when they start declaring doctrines that don't have to deal with the defining of the canon.
At this point I suspect the reason is because the question is unanswerable if viewed through the lens of Sola Scriptura. By necessity, Sola Scriptura rejects the infallibility of the Councils. The viewpoint simply does not allow a good answer. Thus, it becomes a game of attempting to validate the Councils when they agree with a given set of beliefs and rejecting them when they disagree.
"T"radition grew progressively and most of what the RC considers "T"radition today was not "T"radition in the late first and second century.
Or so you say, anyway.
We can debate about bishops, ecclessiology, the Eucharist, credobaptism, symbolic communion, ect. but it would be irrelevant to this discussion since the authors of the scriptures related only one message (multiple applications) which man interprets privately and always has.
It's relevant because Protestantism differs vastly in theology from the ECFs and between denominations. Sola Scriptura leads to nothing but contradictory beliefs, with everybody allegedly being led by the Holy Spirit and the contradictions brushed aside as "non-essential" doctrines to create a false unity. "Non-essential" is never actually quantified of course. The list of what is essential vs non-essential differs from denomination to denomination, person to person.
Even your church interprets scripture (and "T"radition) privately so the argument related to multiple interpretations is a fallacy.
Interpretation occurs merely by reading or hearing words. So of course everybody and every group interprets Scripture, just as they interpret anything else. The reason multiple interpretations can be leveled as a charge against Protestantism is because of the paradoxical nature of Sola Scriptura.
You would prefer to brush aside Tradition as an extraneous interpretive authority and put it on the same level as Protestant confessions or other commentaries. I mean, why not? If you could do that successfully, the entire model for the ancient Churches becomes irrelevant at best and wrong at worst.
But, such a thing cannot be done. Tradition as an interpretive authority stands apart from the Protestant confessions. It is historical fact that the Church was around before the Bible. It is historical fact that it took 400 years to assemble the Bible as we know it today. It is historical fact that it was assembled through Council proclamations. The Bible explicitly tells us there are teachings and events not recorded in the Bible. It also explicitly exhorts adherence to tradition.
Contrast those with the verses used to support Sola Scriptura. Absolutely nowhere in those verses is it explicitly, directly stated that Scripture is the final authority on matters of faith and morals. Absolutely nowhere is it explicitly, directly stated that the everything you need for salvation is in the Bible. 2 Timothy 3:1 6 is about the closest you'll get on that front, but only if you stretch the meaning of the word "work" into meaninglessness.
By necessity, the early Church had to operate on a different ecclesiastical model than Sola Scriptura. There wasn't any Scripture to adhere to! Much of the New Testament wasn't even written until decades after the events of it took place. And even after that, no one was really sure what constituted the full canon. It took the Holy Spirit 400 years to make it sort of clear, and even the canon promulgated by those regional Councils is different from the Protestant canon.
There never was and still isn't any indication that the ecclesiastical model changed after the canon was solidified. God is unchanging. But by necessity, Sola Scriptura has to disagree with and reject the conciliar model of Tradition. At best, the Councils are simply in agreement with (someone's interpretation of) the Bible. At worst, they promulgate heresy.
So, we can either adhere to the ancient model that is backed up by historical evidence, or we can adhere to the model that was invented by Martin Luther in the 16th century.