Preeminence given to one man in church government in the early shows the effect of the tendency of the church to ape the secular government that surrounds it (in this case, Empire), instead of hewing strictly to the Word as normative for church government. The local minister, and beyond that the president of a synod of bishops (which we would equate to a Presbyterian moderator, something like the function of James in Act.15:13), came to be looked to as a man-above-other-men. Eventually, after the passage of a few centuries, lust for power over others led to the elimination of popular election of ministers/bishops or even acclamation of one sent to them, and the city or regional big-wig only appointed such men as he saw fit, who served at his whim.
As for the priesthood, there may have been some linguistic confusion that contributed to the adoption/attribution of the title for Christian pastors. But the bigger problem was not linguistic, but theological. As sacerdotal activity grew in importance (even in the early church), the Christian ministry began to function more and more like a priesthood. But since adopting pagan practices was out of the question, the church began to look back into the OT for precedents. As the church came again "in bondage to the law," the trappings of the Old Covenant began to be adopted and adapted into the church. As interest in the gospel waned, the Galatian heresy (in a new form) gradually overwhelmed the church.