The doctrine of Holy Trinity was coined together by the Church fathers to combat heresies of Arius, Nestor, etc.The doctrine of Holy trinity is not in the Bible, but it's based on our faith in the Bible.
Was Mary the mother of God. No because she would have to be the mother of the Father and the Spirit as well since God is inseparably one
Obviously your church doesn't teach anything about the Divine Economy of our salvation and how the Hypostases act separately yet in harmony.
God does reveal Himself to us as three separate Persons, and these separate Divine Persons act and accomplish different steps in our salvation.
By your logic Christ never left. But He said He would, and that in His stead He would sent the Holy Spirit to watch over us. That's not "possible" in your theology because God is "inseparably one."
The Doctrine of the Trinity IS taught in Scripture, but there isn't a single verse you can go to. You have to take Scripture in its entire context and you get there.
I am aware of the "Divine Economy" of salvation and the roles of the Godhead. I think we've worn the Mary Mother of God thing to death. She is the mother of Jesus per the Bible and that's where I stand.
Jesus is God. God is Omnipresent. Jesus is Omnipresent. He by his very nature is not bound by our elements of space and time.