RobGreetings Mushtaq, in the name of the blessed Jesus may the peace of God rest upon you. As I make a response I will place your topic numbers appropriately.
5) Early Church Fathers
You are probably aware various branches of Christians take different approaches in establishing the reliability of their beliefs. This also applies to the concept of the trinity. For some the fact their church tradition has established a concept as truth is adequate for them. I will return to this approach later.
1) Don’t say three
Apostles Write of Father, Son and Spirit as God
Ignatius 70 – 110 AD a bishop at Antioch who knew the apostles well referred to the ‘Triad’ of God.
Irenaeus 175 – 195 AD a bishop of Lyons, France spoke of “The Son and Spirit as God’s two hands”
Tertullian 220AD was first to use the term ‘Trinity’

We could also frame the Biblical revelation by speaking of the ONE God as existing in THREE ‘personal modes of being’.
Oneness and threeness can also be expressed mathematically without contradiction or illogicality. Consider a cube it has 1 height + 1 width + 1depth 1+1+1 = 3 dimensions but 1x1x1 = 1 cube.
There are many things in nature that are one which we experience in three The sun as a heavenly body, as light, as heat it all in its totality both one and three at the same time. Should we be surprised that we see in nature a reflection of it creator.
1a) Then don’t say two!
Many Islamic scholars claim that the Quran is an eternal book in heaven written in Arabic. This is deemed essential as there was never a time when Allah was without his word. But this is a great difficulty for Islam it presents two eternal entities God and his eternal written word; whereas Christianity has only one eternal entity the one God who exists in three personal modes. Jesus before His birth existed eternally as the Word of God begotten from the heart of the Father.
6) Truth is established from holy books – which was what the Fathers of the church did!
However for a Muslim I am of the opinion that to establish a concept as truth from the original Old Testament prophets and New Testament apostolic scriptures would be far more convincing for you. This is what the notes I sent you pursue.

It is not that I object to the concepts of the fathers of the Christian church but that I personally return to the same source material used by them in establishing the truth of the ‘trinity’ concept. This does open up another line of enquiry which is that of the evidence for the authenticity of the Christian scriptures. I am aware of the Islamic contention that they have been altered but consider your case totally without grounds.
The facts are clear that the original Christian scriptures penned by the apostles and their intimate associates reveal that there is God the Father, God the Son and God the Holy Spirit.
The categories of scriptural evidence for the three modes of God need to be searched out throughout scripture and some of these are given below being the majority of the contents page of my notes (previously sent), which supply many more texts.
(A mode is a way of being as H20 is one molecule but has three ways of existing as that molecule steam, water and ice)

The Name of God in the Old & New Testaments
The Deity of Christ in the Old Testament
God Has an Image or Likeness
Old Testament Appearances of God (frequently in the image of a man) Gen. 16:7-13, 17:1-3, 18: 1-3, 13, 17, 20-26, 33, 19:I, 24-27. 32: 24-30, Hosea 12:2-5; Exodus 3:2-6, 7, 11-18; Numbers 22:21-35; Joshua 5:13-15 & 6:1-2; Judges 6:11-24; 13:3 & 18-22; Isaiah 6:1
Jesus Is The Angel of The LORD
Titles of Yahweh Ascribed To Jesus
Divine Offices of Yahweh Ascribed To Jesus: Father, Creator, Saviour, Judge, Redeemer
Divine Characteristics of Yahweh Ascribed To Jesus
The Works or Activities of Yahweh Are Ascribed To Jesus
Texts About Yahweh Ascribed To Jesus
Spiritual Service Is Exclusively For Yahweh, But Is Rendered To Jesus
Christ, The Messiah is Identified as God in The Old Testament
Old Testament Worship to Christ (Jesus) Implies That He is God
They Will Look on ‘Me’ (i.e. in context ‘Me’ is God) Whom They Have Pierced – who was pierced?
Jesus Taught His Disciples What ALL The OT Scriptures Said About Himself
The Deity of Jesus in the New Testament
Jesus Personal Claims to Deity (I Am = Yahweh) – Jesus Is God & He is Called God – Jesus Is Lord – ‘Greek Kurios’
Divine Titles of Jesus
Jesus is declared to be The Son of God by God, by His enemies, Jewish leaders accused Jesus of blasphemy for claiming equality with God/Son of God
Worship is given to Jesus
Divine Characteristics Are Ascribed to Jesus
The Blood of God
The Deity & Personality of The Holy Spirit
The Trinitarian Formula
The Rock – an extended study of just one of Yahweh’s titles that is ascribed to Jesus

Verses Not In The Original Text Historical Contributors to Trinitarian Understanding & Current Foes of Trinitarianism

7) Unitarians

You have questioned this by reference to Unitarians and Jehovah’s Witnesses (JWs). Such sects have always been an extreme minority just as there are unorthodox Islamic sects. They did not stem from the most eminent scholarship and the JWs have added words to the holly books that are nowhere found in the original text in order to make their case and hide this fact from their followers. JW will not / cannot substantiate the scholarship behind their New World Translation of the Bible. They cannot provide the translators names or qualifications. Try as I may I cannot get them to continue in dialogue with me when I present them with evidence of their faulty translations and request this information.
2) Majestic plural is not proof of Triune God

Here no doubt you are referring to the Old Testament name for God Elohim which is a plural form. You are correct it is not a proof it is only an indication that there may be something more about the nature of God than solitariness. The creation text that refer to GOD (Elohim) creating man however give further suggestion and evidence of something more that a solitary ‘Oneness’ of GOD. We read “Let us (plural) make man in our (plural) image (singular)”. Notice there is a plurality in the creation but a singularity of image – who is this image of God in which man is created? In the New Testament further revelation informs us that “Jesus is the image of the invisible God”.

Mushtaq the weakness in your case is that you have picked out just a few scriptural facts and texts. However to establish scriptural truth you need to harmonise the whole Biblical scripture and accept its teaching.
Metaphors of God are not proof of Triune God

Again you are correct when you speak of metaphors but it is a different matter when the Old Testament prophets and New Testament apostles speak of the appearances of GOD as a man and when the apostles attribute to Jesus the functions that belong solely to GOD (See appropriate scriptural categories above and in my notes)

4) Why does the Old Testament not teach the three – why is it insistent upon the one?
You ask why the three of God is not directly spoken of in the Old Testament (the scriptures of the Jews). I would compare this with the ‘abrogation’ in the Islamic tradition of scriptural understanding. This being the standard means of interpreting the Quran. If a revelation given to the prophet Mohamed changed an earlier revelation then the latter revelation is the superior.
When considering the concept of ‘trinity’ the facts are similar the New Testament trinity theeness concept is not an abrogation of the Old Testament Oneness concept but a development and clarification of that oneness as the unity of the tree modes of the one God. What the Old Testament implies the New Testament makes specific.
The nations of old had many Gods they were polytheistic and the Lord had to establish the truth of Hid Oneness before he could reveal That He was One in Three modes.
The church arrived at this understanding by careful consideration of the whole of scripture and you may confirm it by your own study.
5) Early Church Fathers
To return to the value of church ‘tradition’; the church from the beginning taught that Father, Son and Spirit were each divine and personal. The formulation of statements of doctrine in technical terms to describe this became necessary in the face of heretical teaching originated by the heretic Arius who was only a church presbyter. One means tradition was to appeal to the collective opinion of the leaders of the churches that were established by the apostles, which leaders could trace their succession from the original apostles. This means provides a double check as it were on the understanding of the apostolic scriptures and teaching.
I do not think such a means provides absolute proof but it is strong evidence and the earlier the closer in time the statement by these fathers is to the time of the apostles the greater the likelihood of their reliability. It is in this manner that I consider and value church tradition.