Thank you for your courteous and detailed reply
I would disagree with some of your points though.
P1: All cultures adapt, appropriate and borrow from their cultural environment.
P2: Festivals existed throughout Europe
C: Festivals influenced the behavior and expectations of subsequent generations of the proximate cultures.
We can maybe draw links between gift giving in Late-Antique Roman Christmas and gift giving at Saturnalia, or "lord of misrule" type figures at Saturnalia and medieval Christmas.
But we can also pretty easily see that gift giving can emerge independently, and that in hierarchical societies then 'carnival' periods where normal rules don't apply can serve a social function and thus can also emerge independently.
The thing is gift giving and lords or misrule died out at Christmas well before modern times. You then had periods of alms giving from the upper classes, a tradition of St Nicholas that noted his charity (such as paying the dowries of prostitutes), and then modern style gift giving emerged in the 19th C.
Even if there were influences on the early generations, then these don't seem to have survived in great amounts.
Other things like Yule logs and stuff could just as easily have began as Christian traditions as earlier ones that were kept on. Most of the things asserted as pagan traditions have no real evidence behind them and we just assume that they look "pagan-ish" therefore they must be pagan and have survived since time immemorial.
P1: From its inception Christianity had as a overriding goal the conversion and cooption of all other competing cultures.
P2: The tools available to Christianity were assimilation and conquest.
C: Christianity went hard on assimilation and conquest
That isn't accurate imo, and doesn't really apply a great deal to pre-4th C Christianity.
Once warrior cultures became Christian it starts to contain a degree of truth, but is far too much of an oversimplification to have much use as a general tool of analysis. We can certainly look at specific situations where such things occurred and look at the effects, but on this topic (or any other topic) it says very little of relevance without specifics being added to the mix.
P1: Festivals influenced the behavior and expectations of subsequent generations of the proximate cultures.
P2: Festivals are a tool of assimilation of a culture.
C: Christianity went hard on festivals as a tool for cnversion.
Somethings along these lines must have happened in some places (as in the appropriation of pagan feasts), but the degree to which it happened and its influence on Easter, Christmas, etc. is very debatable. For example the Pope Gregory letter was saying to consecrate new feast days to take them away from the old practices. This is basically the opposite of coopting existing feast days.
Celebrating and promoting aspect of your own culture is not the same as coopting the old culture imo.
We can also draw parallels between some saints and some gods (although the lack of sources can make it hard to identify the extent to which this happened).
I think many people tend to almost assume ancient Christians thought like modern atheists and everything was just a means to a cynical end rather than something that (many of them at least) took very seriously.
Again, it is too much of a generalisation of too diverse and long a timescale to have much analytical value on specific questions.
Would you agree that theological considerations were primary in determining the date, especially through the symbolic connection between Jesus' conception and the vernal equinox, while pagan cultural influences contributed to and helped reinforce and popularize the celebration, particularly in the Roman Empire, where midwinter festivals were already prominent?
For example, I agree with the first part, and the second part is plausible but what would these pagan cultural influences be? What would the evidence be?
There are also plausible arguments that paganism and pagan midwinter festivals played minimal role in reinforcing and popularising it. Maybe they did, maybe they didn't. We can create numerous plausible narratives to explain things. There are all kinds of seasonal festivals that emerged and declined for many reasons.
How would we factor in that Christmas wasn't a particularly big celebration in many places throughout the medieval and early modern period, and much of it is really a modern celebration with modern practices?
What can even be traced from (late) antiquity to the modern day regarding Christmas other than its date? How does this affect the way we should view the question of pagan influences on Christmas?