The simplest way for me to answer to your question concerning the scope of Heid's
Celibacy in the Early Church is to provide the subtitle -- "The Beginnings of a Discipline of Obligatory Continence for Clerics in East and West" -- and to repeat the review found above:
Heid presents a penetrating and wide-ranging study of the historical data from the early Church on the topics of celibacy and clerical continence. He gives a brief review of recent literature, and then begins his study with the New Testament and follows it all the way to Justinian and the Council in Trullo in 690 it the East and the fifth century popes in the West. He thoroughly examines the writings of the Bible, the early church councils, saints and theologians like Jerome, Augustine, Clement, Tertullian, John Chrystostom, Cyril and Gregory Nazianzen. He has gathered formidable data with conclusive arguments regarding obligatory continence in the early Church.
Heid does not advocate a position regarding obligatory celibacy or continence, but merely presents the available historical resources, offering his analysis of them and drawing conclusions. I recall nothing about the "establishment of seminaries and their effect on the cessation of recruiting priests from amongst married men" in the first 100 pages, covering the period up to Nicaea, which is my area of interest.