It doesn’t matter what Sproul or Luther thinks (in so far as them having any special charism from the Holy Spirit). I only cited Sproul because he’s a prominent Protestant minister who accurately presents the Protestant belief in the priesthood of all believers. The only thing that matters is that (special guidance from the Holy Spirit notwithstanding) a person’s teaching conforms to God’s written word.
The Scriptures are God’s Word written. The Holy Spirit will not contradict himself.
That would depend on one’s ecclesiology, and not every tradition has the same form of church polity. Some have an episcopal government, other’s a presbyterian polity, and in congregational churches the members of the church themselves have a role in church governance.
There is God-ordained leadership within the church. We are told in Scripture that the early church chose elders and deacons. We are also told that God has given to the church " apostles, the prophets, the evangelists, the shepherds and teachers, to equip the saints for the work of ministry, for building up the body of Christ, until we all attain to the unity of the faith and of the knowledge of the Son of God" (Ephesians 4).
Of course, the primary role of such leaders is to “equip the saints for the work of ministry” so that we can attain to the unity of the faith and mature enough not to be deceived by false doctrine. God has not placed the church under the authority of a new priesthood. The purpose of church leaders are to empower the saints to be ministers themselves.
One would hope that those who have authority within the church would be guided by the Holy Spirit, but Scripture has told us and history has confirmed that there are wolves in sheep’s clothing and tares among the wheat.
That is why Protestant Reformers believed the church should not give any one man or group of men authority to teach what they think the Holy Spirit might be guiding them to teach at any given moment. That kind of authority should be rejected. Instead, all teachers within in the church should question themselves and look to Scripture for their authority.
Well, ultimately, all of us make those assessments by staying in whatever church we happen to attend. We wouldn’t attend a church whose teachings we did not believe were guided by the Holy Spirit. Whether you are Protestant or Catholic, ultimately you decided that your church was guided by the Holy Spirit.
Scripture is read. If a passage is unclear, you use other parts of Scripture to help interpret it. If it is still unclear, you use reason, knowledge of original languages, and even contextual knowledge of the time period to provide more clarity. Once you have determined what Scripture has to say then you can apply it to the issue at hand.
It’s common sense to me. Why would we need to be told basic rules of reading and interpretation by the Bible?