I respectfully opine, mcq; that you didn’t satisfactorily answer the question. If Sola Scriptura is correct; there would be no need to refer to the ECFs and their opinions on authority.
Authority can be found in Sacred Scripture; and I’m thinking in terms of the Upon this Rock discourse.
In this passage, Jesus gave Saint Peter the headship of the Church as His representative on Earth. That states, unequivocally IMHO; that Saint Peter and his successors have great authority as Christ’s delegate on Earth and defines the Church as His Church and Community on Earth. Break away from the Church and that authority; your community lacks any and all Apostolic Succession, your clergy have no valid Holy Orders, and you’re outside of Christ’s community He established on Earth.
I believe this based on a simple, clear and straightforward reading of the passage; which should be a satisfactory answer for you under the conditions of Sola Scriptura.
Now for your statement of an Apostle is defined by what’s in his heart. How can you, as a fallible human being; see into another man’s heart? How can you satisfactorily establish the legitimacy of his understanding that he receives from God?
What if you’re wrong about your belief about that Apostle and ended up disobeying God as a consequence?
Please, don’t misunderstand me. I believe you Protestants are Christians. Both in personal opinion, because you profess faith in Our Lord Jesus Christ; and in the declared statement of the Magisterium, Protestants are our separated brothers and sisters in Christ, possessing some of the Faith; with a chance of receiving salvation with God’s Mercy.