Your conclusions are simply in error.
I didn't say the Trinity doctrine is a 'mystery in the strict sense' and thus can neither be discovered nor demonstrated by reason. I didn't have to ─ the churches said it for me.
Since 'mystery in the strict sense' is a synonym for 'a nonsense', I can only agree with them. Do you?
The triune nature of The God Head is clearly supported by scripture, both OT and NT.
No it's not.
Jesus is mentioned nowhere in the Tanakh. Not anywhere, not once. If in doubt, ask any Jew ─ it's their book, after all.
And as I said, there was no Trinity doctrine till the 4th century. It was a response to popular pressure to turn Jesus into a god, while not running foul of charges of pagan polytheism.
But it came at the cost of ignoring the plain words that the gospels attribute to Jesus eg
Mark 12: 29 Jesus answered, “The first is, 'Hear, O Israel: The Lord our God, the Lord is one;” ... 32 And the scribe said to him, “You are right, Teacher; you have truly said that he is one, and there is no other but he;
Matthew 20:23 “to sit at my right hand and at my left is not mine to grant, but it is for those for whom it has been prepared by my Father.”
Matthew 24:36 “But of that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only.”
Luke 18:19 “Why do you call me good? No one is good but God alone.”
John 1:18 No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known.
John 5:19 “the Son can do nothing of his own accord, but only what he sees the Father doing”
John 5:30 “I can do nothing on my own authority; [...] I seek not my own will but the will of him who sent me.”
John 6:38 “For I have come down from heaven, not to do my own will, but the will of him who sent me”
John 8:42 “I proceeded and came forth from God; I came not of my own accord, but he sent me.”
John 10:29 “My Father [...] is greater than all”.
John 14:1 Let not your heart be troubled: ye believe in God, believe also in me.
John 14:10 “The words that I say to you I do not speak on my own authority; but the Father who dwells in me does his works.”
John 14:28 You heard me say to you, 'I go away, and I will come to you.' If you loved me, you would have rejoiced, because I go to the Father; for the Father is greater than I.
John 16:23 In that day you will ask nothing of me. Truly, truly, I say to you, if you ask anything of the Father, he will give it to you in my name.
John 17:3 “And this is eternal life, that they know thee the only true God, and Jesus Christ whom thou hast sent.”
John 20:17 “I am ascending to my Father and your Father, to my God and your God.”
To which we can add ─
1 Timothy 2:5 For there is one God, and there is one mediator between God and men, the man Christ Jesus.
Quote Paul as you describe, then after considering context of the verse, we can explore more of what Paul had to say.
Happily:
Corinthians 8:5 For although there may be so-called gods in heaven or on earth – as indeed there are many “gods” and many “lords” 6 yet for us there is one God, the Father, from whom are all things and for whom we exist, and one Lord, Jesus Christ, through whom are all things and through whom we exist.
Philippians 2:8 And being found in human form he humbled himself and became obedient unto death, even death on a cross. 9 Therefore God has highly exalted him and bestowed on him the name which is above every name, 10 that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, 11 and every tongue confess that Jesus Christ is Lord, to the glory of God the Father.
And although Jesus claims to have existed in heaven before his appearance on earth, nowhere does Jesus ever claim to be God.
(In John 10:30 Jesus says 'The Father and I are one' ─ this 'oneness' is explained in John 17:20-23, where Jesus hopes that everyone will be one with the Father.)
