"We’ve known since at least 1872 that the Great Flood detailed in Genesis is a descendant of earlier flood myths from Mesopotamia."
Really need to do a bit of reading about the faith you profess.
December 25th was not chosen at random. The majority opinion falls in the "pagans are already celebrating the solstice with related deities on the day, so why not us Xtians join in and show them our God is really the one being celebrated", a more recent opinion ties December 25th to the supposed date of the crucifixion -- that is nine months earlier. What became a heretical sect of Xtianity, the Donatists in North Africa, were celebrating the Birth on Dec 25th as early as c. 200 CE.
The pagan trappings of the Xmas we all know are almost certainly a direct result of letters from Pope Gregory the Great (c. 600 CE) to missionaries advising them to co-opt the temples and sacred places of pagans and turn them into churches while adopting pagan celebrations as holidays of the saints and martyrs.
Basically, we really don't know because the Church right up to the Renaissance either deliberately destroyed or simply threw out with the trash, any and all conflicting texts. The Nag Hammadi documents were buried by an Egyptian monk who had been ordered to destroy them as they were not acceptable doctrine.