That's fine, and with faith, rather than taking an approach that is just purely logical and scholarly, we as believers in God understand the importance of God's presence in our lives and what we feel God is guiding us to. This as well as basic foundations that discern what type of God we worship i.e is it the God of the Hindus? Is it the God of the Muslims?
What confirms Islam's truth is a very big question that won't be able to be answered in such a simple way.
I have logical reasons, reasons that conform to common sense and the way I understand the world, and personal experience. That actually summarises my initial paragraph in this post.
With that said, and in seeking to answer your final question, I'd first like to say that they possess oneness in the execution of God's Will. They work in eternal harmonisation of God's decree.
The reasons I believe in a Unitarian God are:
1. a. This encompasses what I believe to be an All-Powerful, Perfectly Self-Sufficient, being to be. If we are thinking of a God being self-sufficient, we are talking epitomes here. The epitome of a self-sufficient God for example is one that does not operate in a multi-personhood. But He is instead self-sufficiently One.
When I looked up and prayed to God when I was young, without understanding God as described word for word in the Qur'an, it would appeal to my disposition to worship an All-Powerful singular Person. Go to your churches today and you'll see people can't consistently explain what the trinity is. They'd probably even get stumped if you asked them the wrong argument 'If Jesus is God and Jesus died on the cross, that means God died, how can God die?'. A lot of them would understand the trinity wrongly, in a modalist sense, also. It's just not practically and instinctively understood even after 2000 years let alone a random illiterate, uneducated, non-critical thinking, fruits marketseller in Galilee in the year 30 AD. But the Oneness of God is understandable to all.
1b. The God of the Old Testament does not explicitly deny being a trinity or pentity or anything or that matter, but He does go to lengths to state His Oneness.
I just think that the God of mankind would make known that He is a trinity time and time again, rather than presenting His nature saying things like 'there is no one beside me'.
1c. I don't actually believe Paul believed in a trinity. It's hard to say, but Paul could have believed one of three things.
1ca.The Son was a man only.
1cb. Jesus was a divine being, either being greater than an angel but not God of the O.T, not Yahweh..which makes Jesus being 'subjected' to the Father more understandable.
1cc. Jesus was Lord and that the Father was God. 'There is one God, the Father, and one Lord, Jesus Christ'. This is repeated a lot in Paul's works and strikes me as a splitting of the Shema. However, there is no Holy Spirit influence. This seems to be further evidenced in John 17:3 - I honestly am convinced the author of John (heavily influenced by Pauline theology) didn't believe Jesus was Yahweh.
1d. All we have now are very strenuous interpretations of the gospels, in a way in which are written by authors whom we don't really know. We don't know who the oral transmitters are either nor their competence at transmission, we see massive evolutions of the Christology of Jesus from low to high as we go from Mark to John, whereby people increasingly believed Jesus was a divine being of some sort.
Professor Bart Ehrman is a very good figure to look into for more on this.
1e. When looking at the life of Jesus, all I see is a human prophet. He prayed to God. He used 'God' and 'Father' interchangeably. He never said 'I am God, worship me, God is a trinity', he always stated that he has no power of his own but it all of it was given by the Father. There are many unitarian verses, too. This is a massive topic and I could talk about it at depth. But it's clear to me that Jesus is all too human, and only human.
1f. It's very easy for that generation to believe in things like trinity etc. because they were prevalent in tradition. It was closer to home and readily accepted in their consciousness.
1g. I believe Islam is the true religion, and I have reasons for believing that which I will expound upon if you would like. The Qur'an emphasises God's oneness. The Oneness of God and the belief in that goes beyond just believing that God is unitarian, but it is symbolic of the way we should live, also. Being in God's image as you will.
It's just the amalgamation of these facts working together, I am literally putting my eternal life on the line but 100% confidently in believing God is a Unitarian God.