What is really at play here is the manipulation of people's perceptions of communism and marx to think that Mao, Pol Pot, etc were perfect examples of Marx's ideals, when in reality it's completely false. Just because people do it wrong doesn't mean it's not a good philosophy.
What I'm actually talking about here is the idea of democratic socialism or a capitalist economy which is regulated by the government in such a way that huge corporations are not allowed to oppress the masses. If you want to talk about how the proof is in the pudding, look at places like the US vs Scandinavian countries. The US is heralded for being one of the most economically free nations yet it has enormous rates of poverty, mass incarceration, and use of fossil fuels, and income inequality is ridiculous. All of these are a direct result of unregulated capitalism. Poverty and income inequality are made possible by keeping the poor in their place with an absence of a higher minimum wage among lots of other things, while corporations get tax breaks from politicians. Mass incarceration is fueled by the massive waste of time and money called the War on Drugs as well as for-profit prisons, giving an incentive for the government to incarcerate people and basically make them do slave labor for extremely low pay. The use of fossil fuels is symptomatic of capitalism's inherent drive to produce, and insofar as renewable energy is not the most cost effective at the moment, big oil companies would much rather produce as much energy as cheaply as possible without caring what happens to the environment.
Capitalism isn't perfect, and I haven't seen anyone on here advocating a pure libertarian economic system, which is what you are describing as capitalism. There is an idea called The Tragedy of the Commons that says where there is a shared resource (in the original example a pasture) but an individual benefit from using it (originally grazing individually owned animals on it) it will tend to be overused to the point that it is useless to everyone. This concept can and has been expanded to consider all of earth's natural resources as a Commons, the idea being that if I dump pollution into the air I benefit more from getting rid of waste cheaply than I am hurt, because the benefit is concentrated but everyone on earth suffers, so the negative effect is spread out. The proper role of government in economics, in my opinion, is to prevent this kind of thing. It will be a never-ending debate as to what exactly that looks like, but that's ok. There are limits to capitalism, and a good government will regulate to some extent, but it is true that when people are acting in their own best interest they work harder and more effectively than they do if they are just working because that's what you do. We're selfish that way.
Plus, you also talk about how capitalism allows for the most acceptance, but what about the image of the black male thug that has been produced by society as a result of continuing cycles of poverty and mass incarceration? Or what about the fact that slavery was inherently tied to the capitalist drive for unregulated labor? Or the commodification of various identity politics movements by capitalism? Or the production of the ideal body image of women by clothing companies? There are many example of how a purely capitalist society does not create acceptance. However, it's not even a question of acceptance anymore as the way oppression is manifested nowadays is in societal structures, which capitalism alters in such a way that people experience lots of hard times, most of which are linked to poverty. Now, let's look at Scandinavian countries. They have what most would consider to be a socialist economy, yet they do not have high tensions with other countries, they don't have a lot of corruption, and they have one of the highest happiness rates in the world. There's your proof in the pudding. It also disproves your tribalism hypothesis since Scandinavia was pretty much the origin of tribalist conquest, with the Vikings a long time ago.
The very fact that you recognize these as problems is an indicator of how far we've come. I'm not going to argue that these things are good by any means, but through gradual change things have improved a lot.
As to the Vikings and Scandinavian countries: first, I think that saying that the Vikings originated tribalism is just simply not true. The Vikings were active around 1300-1000 years ago, roughly the time that Mohammed and early Muslims were conquering the Middle East, and long after the Romans, Greeks, Babylonians, Persians, Egyptians, Israelites, and early Chinese dynasties, to name a few, were doing the same thing. Scandinavia now is unique. You are correct that they have very stable, prosperous, and socialist culture. My brother and sister spent last fall in Denmark, and their observation was that "the only thing not homogenized in Denmark is the milk," by which they meant that people in a given country by and large come from very similar backgrounds, socially, economically, and genetically, and everyone understands that they are part of a larger whole. They mentioned specifically the health care system, which has functioned just fine for quite a while now, but that recent immigrants are putting a strain on the system. Not because there are more people, but because immigrants don't have the same sense that "if I go to the doctor for a stubbed toe, my friend across the street will pay for it." There is a sense of communal responsibility that is lacking in the United States, and probably in most places, that is necessary for socialism to function.
E: Also, the communist countries today that people often reference (China, NoKo) are not communist. China is "communist" because it is technically led by the communist party, but they trade freely with other nations and have a booming capitalist economy. The reason why living conditions are bad in some areas of China is because of the huge amounts of production and capitalist exploitation that takes place, which is actually a result of capitalism, not communism. See, just another example of the subtle ways our perception is tainted by capitalist structures. And North Korea is really just fascism, I doubt there's a single thing in the Communist Manifesto that Kim Jong-un actually abides by.
I believe you are correct in estimating just how technically "communist" China and North Korea are, however the fact of the matter is that they still use Marxist rhetoric to justify their policies, and China at least did start out explicitly trying to implement a Marxist system. The system that is in place in China is corrupt, whatever it is, and it is certainly not properly regulated capitalism. Even with the problems, though, and there are many, there are not millions of people being killed, so I count that as better than when they were really trying to implement Marxism.
Yes, communism in its pure form is simply an economic system. Neither communism nor capitalism has anything to do with gun ownership or gay rights.
You really can't separate things this cleanly. When the United States was fighting the Soviet Union during the Cold War, Whitaker Chambers wrote of communism in his autobiography
Witness:
"It is not new. It is, in fact, man's second oldest faith. . . . Like all great faiths, its force derives from a simple vision. Other ages have had great visions. They have always been different versions of the same vision: the vision of God and man's relationship to God. The Communist vision is the vision of man without God. . . . it has taken the logical next step which three hundred years of rationalism hesitated to take, and said what millions of modern minds think, but do not dare or care to say: If man's mind is the decisive force in the world, what need is there for God? Henceforth man's mind is man's fate."
You might dismiss him, as a fundamentalist nut, but I would point out that in the years after that it was well enough understood that we were a Christian nation fighting the godless communists that in 1954 the words "under God" were added to the pledge of allegiance.
