The one thing we can be sure of is that "ETHNOS" also means tribes in the Greek
HOW may we be sure of that? As far as I can see the one thing we CAN be sure of is that tribe is by no means other than context the "preferred" translation and in fact in the KJV ethnos is not translated as tribe even once according to Young's Analytical.
......and why it was translated "ETHNOS" instead of "PHULE" remains a question.
Could you please rephrase? What is the antecedent of "it"? Are you saying we can know what the original Aramaic or Hebrew Text of Matthew was?
That is the tradition.
The Catholic Encyclopedia says the following:
Ancient ecclesiastical writers are at variance as to the date of the composition of the First Gospel. Eusebius (in his Chronicle), Theophylact, and Euthymius Zigabenus are of opinion that the Gospel of Matthew was written eight years, and Nicephorus Callistus fifteen years, after Christ's Ascension--i. e. about A.D. 38-45. According to Eusebius, Matthew wrote his Gospel in Hebrew when he left Palestine. Now, following a certain tradition (admittedly not too reliable), the Apostles separated twelve years after the Ascension, hence the Gospel would have been written about the year 40-42, but following Eusebius (Hist. eccl., III, v, 2), it is possible to fix the definitive departure of the Apostles about the year 60, in which event the writing of the Gospel would have taken place about the year 60-68. St Irenæus is somewhat more exact concerning the date of the First Gospel, as he says: "Matthew produced his Gospel when Peter and Paul were evangelizing and founding the Church of Rome, consequently about the years 64-67." However, this text presents difficulties of interpretation which render its meaning uncertain and prevent us from deducing any positive conclusion.
Papias (Eusebius, H.E. 3.39.16) "Matthew collected the oracles (ta logia) in the Hebrew language, and each interpreted them as best he could."
Irenaeus, Adv. Haer. 3.1.1 "Matthew also issued a written Gospel among the Hebrews in their own dialect while Peter and Paul were preaching at Rome and laying the foundations of the church."
Origen (Eusebius, H.E. 6.25.4) "As having learnt by tradition concerning the four Gospels, which alone are unquestionable in the Church of God under heaven, that first was written according to Matthew, who was once a tax collector but afterwards an apostle of Jesus Christ, who published it for those who from Judaism came to believe, composed as it was in the Hebrew language."
Eusebius, H.E. 3.24.6 "Matthew had first preached to Hebrews, and when he was on the point of going to others he transmitted in writing in his native language the Gospel according to himself, and thus supplied by writing the lack of his own presence to those from whom he was sent."
Epiphanius (ca. 315-403), bishop of Salamis, refers to a gospel used by the Ebionites (Panarion 30. 13.1-30.22.4). He says it is Matthew, called "According to the Hebrews" by them, but says it is corrupt and mutilated. He says Matthew issued his Gospel in Hebrew letters. He quotes from this Ebionite Gospel seven times. These quotations appear to come not from Matthew but from some harmonized account of the canonical Gospels."
Jerome also asserts that Matthew wrote in the Hebrew language (Epist. 20.5), and he refers to a Hebrew Matthew and a Gospel of the Hebrews-unclear if they are the same. He also quotes from the Gospel used by the Nazoreans and the Ebionites, which he says he has recently translated from Hebrew to Greek (in Matth. 12.13)