In answer to points 3, 4 and 5. The first council of Nicea was, in part, formulated to resolve the problems of the Arian crisis, which arose because of logical contradictions in the doctrine of the Trinity. These problems had plagued Christianity for around 100 years, ever since Tertullian formulated the idea in one of his works. These problems did not appear to exist before Tertullian. This strongly supports the idea that the Trinity is a late second century AD idea, and not original to Christianity. Looking at the Gospels, neither Mark, Matthew, nor Luke have anything that is definitely Trinitarian in nature. The idea of Jesus being tested in the desert and in the garden of Gethsemane demonstrate the first Christians did not believe Jesus was of the same nature of the Christian god. You do find the three (father, son, ghost) mentioned together before Tertullian, but that does not mean they were regarded as 3 in 1. The Gospel according to the Hebrews, from the early second century AD had the three together, but as a family unit, with the spirit as the mother of Jesus.
