Why does the Coptic Church interfere in the internal affairs of the Ethiopian Church? Would it be possible for the Oriental Orthodox to hold an ecumenical synod to resolve these issues? , Besides, not all the bishops in Ethioia were Egyptian Copts. For example, Mar Frumentyos, aka Abba Selamae was Syrian Orthodox. Also, what exactly were the missionary activities of the Coptic Orthodox?
The Coptic Church does not interfere in the internal affairs of any other church nor does she get involved in the political affairs of any country. The Coptic Church adheres and points to the right teaching she has received from the Apostles and the Early Fathers. St. Clement of Rome (1st Century), in his "Epistle to the Corinthians", stressed that the presbyters cannot be deposed by the members of the community because authority is not bestowed by them. He wrote, "From land to land, accordingly, and from city to city they (the apostles) preached and from their earliest converts appointed men whom they had tested by the Spirit to act as bishops and deacons for the future believers. And this was no innovation, for a long time before the Scriptures had spoken about bishops and deacons; for somewhere it says: 'I will establish their overseers in observance of the law and their ministers in fidelity'. Our apostles too were given to understand by our Lord Jesus Christ that the office of the bishop would give rise to intrigues. For this reason, equipped as they were with perfect foreknowledge, they appointed the men mentioned before, and afterwards laid down a rule once for all to this effect: when these men die, other approved men shall succeed to their sacred ministry. Consequently we deem it injustice to eject from the sacred ministry the persons who were appointed either by them, or later with the consent of the whole Church, by other men in high repute." This is unfortunately, what happened in the case of Abune Tewophilos.
From the beginning the Ethiopian Church was affiliated with See of St. Mark of Alexandria. In the second quarter of the Fourth Century, King Ezana of Ethiopia appeared to the leading figure of the Church organization. (352-359 A.D.) This was a period of time of the radical shift in Church life to establishing Christianity as the State Religion of Ethiopia declared by the king. The king himself initiated the historical link between Aksum and Alexandria when he sent out Frumentius to St. Athanasius, the patriarch, in order to send a bishop to his kingdom. St. Athanasius, on his part, after learning the need for a spiritual leader to that land, consecrated Mar Frumentius, the private secretary of the king, after the name "Abbe Selama" as the first Bishop of Ethiopia. After the death of Abbe Frumentius, Egyptian bishops were appointed to head the Church; until 1959 when a complete independence was granted.
Yes, the oriental churches can have a council to discuss these issues but we cannot call it ecumenical since the word ecumenical means it involves all the churches in the whole world. As a matter of fact the Coptic, Syrian, and Armenian Church of Lebanon meet regularly in order to unite their voices as you were saying.
The Coptic Church started a big mission in Africa and established many churches there. Now we have two bishops in Africa. For further reading, I recommend Bishop Antonios Marcos' book "the Church in Africa."