Well, you can't have it both ways. You seem pretty knowledgeable so why are you so insistent that free trade is voluntary even though that would conflict with your other opinions?
If it's voluntary, that little quirk you mentioned about Keynesianism, wins: there's no loss from changing your mind 100 times.
You are ignoring friction. There is loss from moving things around. Even if the things don't move, just updating computer records of ownership has non-zero costs. Trade is beneficial when the benefit of the trade exceeds the friction cost.
But if markets are like little
heat engines that only go one way because energy is taken out of the system, then there's nothing voluntary about resources pushing from the hot side to the cold side.
Your analogy doesn't really fit. There is only one "heat", but there are many, many "resources". When one resource is pushing from hot to cold, another resource is flowing in the exact opposite direction, but also from hot to cold,
in terms of that resource.
The voluntary part comes from being able to decide for yourself which resources you have that you want to trade, which resources you want, with whom you want to trade, when you want to trade, and at the exchange rate you want. Of course, the other sides are free to make the same determinations, so "voluntary" doesn't imply that you get everything you want for free. Money vastly simplifies this process by abstracting away one side of nearly all of those trades.
The gains from the system come when you realize that for various reasons different people are able to provide different resources at various costs. Some soil is excellent for growing corn, some soil is excellent for growing watermelons, some people are excellent at producing car engines, some people are excellent at producing robots, etc. You can grow your own food and raise your own livestock, but someone out there can probably do it more efficiently, so if you are good at something else, you two can trade and both benefit. You get your food cheaper than if you grew it yourself, and the farmer gets shoes, or tractors, or kitchen cabinets, or whatever for cheaper than if he did it himself.
You could run with the with the heat engine metaphor and argue that it's a huge chaotic system, e.g.: MtGox takes the fees they earned and pay for something else, which is part of another market with energy getting sapped by other middlemen, ad infinitum.
You see middlemen, I see enablers. Mtgox provides a marketplace with access to more buyers and more sellers. Without that marketplace, you'd have fewer choices for trade, meaning that you'd get less when selling, or pay more when buying. In other words, they reduce friction. If you feel that the fees they charge for that service are too high for the value they provide, you are free to sell elsewhere.
But the whole thing relies on the energy of the workers on the hot side being constrained as much as possible, before they get the freedom they want on the cold side. This is probably why Capitalism seems so unstable: everyone wants to use less energy, so people start cooperating instead of competing, then they form monopolies, then those monopolies take on political characteristics, as I think Odrec suggested.
Typical Marxist nonsense. Go read some better economics books. Hazlitt, Mises, Friedman, Hayek, etc. Capitalism and free trade in general do not rely on artificially hindering anyone else.
One thing that Marx was right about is that people have a natural tendency to try to use government to protect their interests*. There are two ways to approach that problem. One of them works quite well, but requires diligence. The other is a complete and utter failure every time.
One approach is to limit the power of government so much that it is unable to provide anti-competitive benefits. In this scenario, it doesn't matter that someone may attempt to use the state to protect their position, because the state is powerless to help them. This method works well, but requires that people not give in to the temptation to allow creep.
The other approach is to make government more powerful. This is always and everywhere a failure. The government that you can use to tell someone else what to do is also the government that someone else is using to tell you what to do. People don't like being told what to do, so the trick here is to either get everyone dependent on the government so that their survival appears to depend on the system growing, or fill up the mass graves until people are too scared to speak out.
*
Marx was pretty good at observation. Less good at deduction. Terrible at projection. Not that it matters, no one has actually read his stuff in years, they mostly just repeat things attributed to his school.