After studying the entire scope of Near Eastern history I now have some confidence in making generalities. (Admittedly there is much room for improvement.)
There was, as Atwill notes, a type of integrity in the genre of 'sacred historty' writing. The authors could not just invent things arbitrarily. They had to have some basis in real history or real astronomy as they understood it.
The mythological cycle of Jesus originates with the god Horus (and his counterparts in other regions and in cosmic conceptions). The root myth of Horus was modified by a large number of later persons who played that 'divine' role as 'incarnations' of that god. In the times of the pharaohs, it was common for multiple princes to be designated for the same role and at the same time. That tradition is reflected in the Gospel accounts.
Also in the time of the pharaohs, real persons that played that part sometimes also assumed the roles of various other gods, causing the root myth of Horus to further evolve and become more complex. By the end of the pharaonic era, it was thought possible for a single Horus king to combine the roles of all the gods - to become as the composite god Amun.
The 'root and branch' motif is actually a metaphor for kingly succession. Ancient kingship was likened to a towering tree. Because of inbreeding a dynasty could only be expected to last about four generations. It would then be necessary for a collateral line ('branch') to emerge from the same root/trunk. It was often considered necessary to kill royal rivals, but poor judgment to extirminate their descendants with them. The dominant king was expected to leave the root in tact. Otherwise when the ruling king's branch died out it would lead to the death of the entire 'tree'.
I don't think we are so far off in our thinking about these things. You perhaps emphasize more abstract and astrological notions of Jesus. I'm trying to determine the contributions that specific royal persons made to the aggregate myth.
© Charles N. Pope, US Library of Congress. All rights reserved.