I'll ignore the low blow and redirect this back to my point about John Calvin and OSAS.
I didn't mean it as a low blow. That is literally how Ehrman has articulated the Bible. If you are consistent with that kind of epistemology, you'll follow it right out of Christianity altogether.
Is John Calvin's teaching closer to what Jesus, the Apostles or the early church fathers taught, or is it based off of his understanding of the church of his day, his understanding of the text and his personal context? In history, there were 1500 years of church tradition before Luther and Calvin, so was all of that theology and practice incorrect until the Reformation?
I recommend that you read the Institutes. "Calvin vs 1500 years of tradition" is both anachronistic and inaccurate. The idea that the first 1500 years is a monolith is a myth. The fact that you have multiple churches claiming to be "the church of the first 1500 years" is all the evidence you need to see that.
Your first question, the answer is a bit of both. I don't see a dichotomy there. Second question, no, not
all the theology was incorrect, but it's imprecise to flatten out the first 1500 years as if there were no doctrinal developments, schisms, controversies, political maneuverings, etc.
In addition, one of the fundamental flaws of sola scriptura is that not all of the traditions of the church are contained there. What of the 'doctrine of the Apostles' as mentioned in Acts 2:42? What are the things God spoke directly to the prophets? What of John 21:25, where not even all of the acts of Jesus himself were recorded?
If a church contains a tradition that is not in Scripture, then the "fault" is in that church and not the Scripture. This is not a "flaw" or a bug, but a feature.
Acts 2:42: And they were continually devoting themselves to the apostles’ teaching and to the fellowship, to the breaking of bread and to the prayers.
Not sure what you see in this other than what it says. In context, the Apostle's teaching = the Gospel.
What of John 21:25, where not even all of the acts of Jesus himself were recorded?
How is this an evidence against Sola Scriptura? Are these unrecorded things we need to know? Does Oral Tradition tell us what other things Jesus did? Why not read the previous chapter?
John 20:31 but these have been written so that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name.
However, achieving a form of fullness in one's walk with God means being a part of the church. Not that it is in itself necessary for salvation, but life is meant to be lived in community. St Paul commands believers to assemble (and not just gathering for 1.5 hrs every Sunday).
No one's arguing against this.
Where things get murky and waters are quite muddied is when folks start splitting hairs about doctrines and ideas that were never taught by Jesus Christ or the Apostles. My gial is not to be hypercritical but just look at the state of Protestant churches in North America. Something is clearly amiss...
If Protestants split hairs over what the Bible teaches, how much more do you think non-Protestants split hairs over what the Bible doesn't teach? As I said above, look into the history of the schisms. The idea that the first 1500 years was the Church living in static harmony is a lie.