Is the official ecclesiastical opinion for an Orthodox priest or bishop only established by Church council or by revelation of the Holy Spirit followed by submission to Church authority? If this tabernacle of David restoration (Amos 9:11) is happening, what does Orthodoxy have to say about it?
The answer to your first question requires some knowledge about the Holy Sacrament of Priesthood in Orthodox Church.
Priesthood (the rank includes the orders of deacon, priest, and bishop) is a Holy Mystery in which the bishop lays his hand on the head of the elected candidate in order for the Holy Spirit to descend on him. A certain prayer is prayed on this occasion, then the bishop gives him one of the priestly ranks by which he is granted authority to perform church ministry, designated for his rank be it the Holy Sacraments, doctrine or others.
This Mystery had been instituted by our Lord Himself when he chose the twelve disciples. --"He called His disciples to Him, and from them He chose 12 whom He named Apostles” (Luke 6:13).
These Twelve were sent out by the Lord and commanded to preach saying "the kingdom of heaven is at hand. Heal the sick, cleanse the lepers, raise the dead, and cast out demons" (Matthew 10:5-8).
Other Biblical references include: Holy Gospel of St. Matthew 18:18, Holy Gospel of St. John 20:22-23, Holy Gospel of St. Luke 22:14-20.
The Coptic Church believes priesthood to be a Divine call, a Divine choice, an appointment, and selection. Simply appointment and selection entails that if a servant is recommended for priesthood, by the elders and congregation, then he should be recommended because of good manners, successful service and good deeds. He should be well-knowledgeable in doctrine, meek, comforting, merciful, and married according to the Law of the Holy Gospel.
This is only a very brief overview of the Mystery of Priesthood. Several books are written on the subject of Priesthood in the Coptic Church. One well-written book is "Sacrament 7: Priesthood" by His Grace Bishop Mettaous.
The second point in your question regarding “revelation of the Holy Spirit" speaks to "spiritual knowledge". The definition of the concept of "spiritual knowledge" in the Coptic Orthodox church is "knowing and experiencing the truth of God and salvation through the Lord Jesus Christ." Spiritual knowledge is a) frequently identified with Christian doctrine; b) is applied to the spiritual meaning of the Holy Scripture, and c) refers to mystical and contemplative knowledge, not merely intellectual knowledge of God. The aim of "spiritual knowledge" and effects are to enhance man's responsibility, to aid in discernment of good and evil, and to lead people to God. Many Scriptural references are provided to support such a definition. (Luke 12:47; Luke 48; I Corinthians 13:2; II Corinthians 4:6; Ephesians 4:11-16).
It is clear from the Holy book of Galatians 1:11-2:2 that any revelation should be submitted to the church authority and hierarchy for checking.Otherwise many can be deceived.
The verse in the Holy Book of Amos 9:11, is a prophesy about the church of the New Testament that had been established on the day of Pentecost. It still exits until today and will exist until the second coming of the Lord. With this concept in mind, it is easy to understand how the Coptic Orthodox faith has remained unchanged for more than 2000 years and can be readily traced to the Apostle and Holy Gospel Writer St. Mark himself. The Orthodox Church follows the unchanged faith of the Apostles and early Church Fathers. The Orthodox faith has remained and will continue unchanged.