The problem with your suggestion is that while seemingly arguing for an evolutionary, trial-and-error approach, you are actually being ideologically dogmatic.
Just like a successful market-based system is a product of evolution and tinkering, so is a successful regulatory environment. States, governments, laws, and regulations also evolve. You state that people cooperate through a free market, which is true (not only by competition however, because they do actually a lot of very straightforward, non-competitive, er - cooperation), but people also cooperate through their political and other social institutions.
Thus, the parent poster is quite correct: instead of being dogmatic (the state must do X; the state must not do X; Y must always be left up to the market; Y should never be left up to the market; etc.) we should try different things, and see what works. A lot has already been tried and we have good track record of what has been successful and what has failed; but there are many more other things to try. Giving definitive statements like yours - "The most robust and humane way to shape society is Capitalism" is simply, wrongheaded (not to mention that "Capitalism" can be defined in a myriad of ways and has many forms in practice).
Finally, your idea that "the only way for their to be an objective measure between 2 individuals is for those individuals to agree in advance about the rules of their future interaction" is also wrong. The only thing which defines "good parts" and "bad parts" of a social system is success of failure in the real world. Theory and abstraction in this case count for almost nothing. Empirical data is all that matters. The conclusion is that, in order to promote evolution that will eventually produce successful modes of social organization, we should have a social, political, and economic environment that grants a large degree of individual freedom, giving people the ability to tinker and try things out. However, we should also have a democratic-type organization which allows people to rotate different groups in government with different ideas, to see which kind of government policies work. You will find that the world's most successful countries (e.g. Switzerland) follow this approach. Incidentally, Switzerland is both an efficient low-tax state with much individual economic freedom, while also being a quite regulated society in many aspects (with those regulations, due to extreme decentralization and the use of referenda, usually being based on a wide social consensus).
The reason that market-based systems (as opposed to centrally planned economies) and democracies or similar politically-decentralized states (as opposed to centralized dictatorships) have been in the long run the most successful is precisely due to the tinkering and evolution that is inherent in them, and not due to grand theories of economists. Even the rules which you cite were never derived in reality by being agreed to in advance, but rather evolved as people tried different ways of interacting with each other.