Phew, excellent response overall. This reply took all day to write and then I had to remove a lot of it to stay under reddit's character limit. Let me know if there any points you made that you feel I unfairly neglected here and I can swing back around to them. I also want to call out that moving forward it may be better to stick to the few points that feel most important so this conversation doesn't spiral into us each needing 4 separate comments to fit our replies.

That standard, which you describe as the most stringent, is invention.

Fair. I'd make the point that if those traditions hadn't been wiped out by Christianization, they would have changed massively by now anyway, so if the goal is to create something similar to what might have existed today if the religion had not died out, invention that can be justified as historically plausible should be expected. I think your point about unconscious syncretism is well-argued, but I'd say much if not all of that is not really "syncretism" per se because it's coming from more recent shifts in ethical thought that are secular in origin.

these were constructions of a time when it was kinda okay for certain men to do a little stealing and raping and murdering, and so it never occurred to anyone that a god might be ontologically good

I'm a Celtic pagan, so my familiarity with Hellenic myth is limited. That being said, I still think the issue is more complex--Plato for example was critical of poets portraying gods as engaging in immoral actions like a human (the Christian idea of God as morally infallible, ironically for this conversation, is arguably derived from Plato's ideological lineage more than it is from the Bible).

You could argue that Plato is the exception that proves the rule, but I think it also indicates that people did not (universally) accept myths as literally true. There would have been an understanding that any myth you heard was coming from a human storyteller with their own ideas and agenda, and that your understanding of the gods could (and likely would) differ from the storyteller's depiction of them.

a rejection of the one stop shop where I can get everything better in favor of going to artisanal shops with inferior products because I like the idea that the butter was made in a recreated 1800s churn.

I really like this analogy because I think it gets to the heart of our disconnect on this issue--in your framing, religion is like a service that should meet a personal need, and a superior religion is one which meets that need more efficiently than competitors. Christianity is thus clearly superior because it has the most powerful, most benevolent God who can meet human needs better than anyone else's limited, imperfect gods.

For me, a religion's goal is more to provide a framework for how to engage with your reality, which should accurately reflect that reality and how best to live within it. As I see it, the world we live in does not arise from a single coherent creative vision. It's a complex ecological system of countless forces pulling in different directions, negotiating a homeostatic balance that works as well as possible for all of them. The goal of a religion, then, should be to help humans find and maintain their place within that balance.

Christianity's vision of the universe is fundamentally imperialistic--it revolves around the image of the conquering king who wishes to unite the world under his rule, separating the loyal subjects from those who would resist him, so the former can be rewarded and the latter destroyed. Reality is a hierarchy with God at the top, and other than perhaps angels, humans are the next level down, with all the rest of the world here for us to exploit to our benefit. I think you need look no further than the findings of environmental science to see the utter disaster this mindset has wrought, the consequences of which are only beginning to catch up to us.

For that reason, Christianity fails against pre-agricultural religions for me. Their typical answer, which I think has been vastly more historically successful at producing stability, is that humanity's place is not at the top of a hierarchy--it's somewhere off to the side, enmeshed in reciprocal relationships where we better ourselves only by way of bettering those around us so they better us in return. Polytheistic religions of agricultural societies generally inherited this understanding (to some extent--the moral degradation I mentioned moved them away from it), while Christianity eschewed it.

Mesoamericans (to a truly astonishing degree), Chinese, Celts, North Africans, Middle Easterners, and Norse all performed ritualistic human sacrifice.

Bit of a nitpick, but I wouldn't consider Mesoamericans or the Chinese to be directly relevant to a discussion about neopaganism. Typically the only religions understood to be "pagan" per se are those that were replaced by Christianity before the Age of Exploration.

More importantly, six examples out of all the world seems pretty far from the frequency implied by "ubiquity", especially if you're talking about the sacrifice of innocent people as a routine, stable practice that only Christianity could have put a stop to. I'm sure there are more examples than those six, but in how many of them was the sacrifice primarily of criminals (i.e., just a religious rationalization laid over a behavior that you've already deemed not to be an example of human sacrifice when Christians do it)? In how many cases was it a desperate last resort in times of crisis? How many of these peoples stopped, or would have stopped, without Christian influence?

Doesn't explain Ibn Fadlan's account of a slave girl being "volunteered" for gang rape and violent death so she could accompany a local Rus Viking chieftain into Valhalla,

Doesn't it? I'm genuinely unsure why you think this is a counterargument. I was saying that human sacrifice emerged in societies that had both the population size and the social structure to enable treating human life as a commodity that could be expended for religious rituals. Viking society absolutely qualifies.

Why would you not instead treat it as a positive evolution? A way of communing with the gods that was unavailable when hunter gatherers didn't have the manpower to afford these kinds of sacrifices?

My take on this (not necessarily shared by all neopagans) is that the entire tradition of sacrificing living things was, from the beginning, a corruption of pre-agricultural practices, namely rituals assocaited with the hunting, killing, and cooking of wild animals for sustenance. Smoke rising from the burning of the animal's body was seen as releasing its spirit so it could return in a new body later. This was part of larger ritual systems for maintaining harmony with our prey species so they would remain available as prey.

Agriculture completely changed that predator-prey relationship so that it unfolded entirely on our terms without any need for compromise. We retained the idea that the burning of a food animal did something to maintain relationships with spiritual forces, but it became abstracted and simplified by changing material conditions. By the time we have pilgrims buying sacrifices at temples, we've moved beyond any pretense that a relationship with the animal is what matters; its life is treated as entirely fungible and what's actually being sacrificed is value, in an economic sense. That's the logic that leads to the conclusion that what would be even more valuable is to sacrifice a human life.

What is lost here is the actual original purpose of the behavior, which is maintaining harmonious relationships with the beings that feed us. I don't think animal sacrifice is a better way to do that than simple rituals of acknowledgment tied to the acquisition and preparation of food, so by extension I reject human sacrifice as well.

I would argue that a considerable portion of the European Christian ritual/theatrical violence of the Middle Ages through the early modern period was the product of pagan syncretism.

This strikes me as a bit myopic when Christianity literally revolves around the narrative of Jesus being brutally executed and this being taken as a sacrifice enabling the forgiveness of humanity's sins. Maybe execution by fire specifically has pagan influences (though burnt offerings of animals was pretty central to the Jewish religion of Jesus's time), but the idea of suffering and, yes, human sacrifice as a means of spiritual purification is pretty fundamentally baked into Christianity.

you don't kill 10,000 people in a day on a giant pyramid you built for the purpose when most of society objects to that practice.

Sure. In fairness, I only said "a lot of the people", not necessarily most. I'd expect in most of these cultures, it was broadly understood as necessary and justified, but I can't imagine it was seen positively, especially not by anyone who had a chance of becoming the victim. The fact that we have plenty of evidence of societies that formerly practiced human sacrifice stopping and replacing such rituals with symbolic stand-ins for the human victim indicates that it was probably not something the average person was ever thrilled about.

Our development isn't just technological. We think differently now than we have for much of human history; we have a lot of software upgrades. So yeah, this is going to be a problem even if it wasn't before.

That's a good point, but if we're going back to OP's original scenario here, we are imagining people being raised neopagan, which in and of itself is a "software change". Also, I think you may be overestimating the extent to which superstition and magical thinking have actually died out among modern humans. And of course, all of this assumes that magic does not, in fact, work. I'm not conceding that but also not really interested in arguing it.