I wonder if you have considered the trinity in light of the anthropomorphic nature of religion and politics previous to and concurrent with biblical times. In short wouldn't this place Jesus, if he did exist, on a par with the Roman Emperors, self declared gods incarnate? The experience of the Jews, in all the societies in which they lived, was of gods that could incarnate at will and walk the earth with the common man. Here, in Jesus, was a God incarnate who, unlike the emperor, was a man of the people,a man for the people.
Yes, when you consider the times, it was not that unusual for a man to be considered God incarnate. As you mentioned, Roman emporers made that claim. What is unusual is for Christians to accept this view, while carrying the Old Testament that says there is one God. Of course many accept the idea of 3 beings that are all God, but think that the concept of the trinity explains it. I discuss that issue at 3 Gods?