Even if there were remnants of earlier polytheistic or henotheistic beliefs in isolated Jewish communities, they were not representative of mainstream Jewish belief in the first century, ...
I would not have expected a new sect to arise from "mainstream Judaism". The disenfranchised and disillusioned are the creative ones.
You compare the understanding of Jesus with the way readers might understand the gods of Plato or the dramatizations in Greek mythology, implying that Jesus could be a fictionalized character. However, this comparison is flawed. The Gospels and early Christian writings are not presented as mythological allegories or philosophical treatises, but as historical accounts of events that occurred in specific times and places.
"Fictionalized" i not the word I would generally use. I understand for early believers, the Christ was just as 'real' as they believed his Father was. Or for that matter as real as Dionysus was for Greeks.
As you seem to acknowledging, the Gospels (first of which Mark was possibly a play) it seems were the key in the shift from an ethereal Christ to a guy walking around Judea. That was my point in my last comment. Euripides (and Homer of course) popularized religious concepts (gods) through dramatization. The effective representation of the god Dionysus (that to the philosopher represented freedom from oppression and conformity) through literature and plays cemented his image as a god of the people and endeared him to them. The power of myth. The sheer amount of such stories that were written and continued to be written for hundreds of years proves the popularity of such stories. You probably regard most of those stories as fictionalizations. I see them as dramatizations with an allegorical meaning or didactic motivation.
Earlier I posted an excerpt from the Asencion of Isaiah that stressed that the drama was invisible. That the Christ/Logos/Son took the appearance of flesh as part of his disguise as well as in anticipation of being crucified on a tree.
Ascension of Isaiah 9: 13. Nevertheless they see and know whose will be thrones, and whose the crowns when He has descended and been made in your form, and they will think that He is flesh and is a man.
14. And the god of that world will stretch forth his hand against the Son, and they will crucify Him on a tree, and will slay Him not knowing who He is.
15. And thus His descent, as you will see, will be hidden even from the heavens, so that it will not be known who He is.
16. And when He hath plundered the angel of death, He will ascend on the third day,
It is only reasonable to question when this shift from an ethereal unrecognized Christ to a famous bloke doing miracles in Judea took place.