: If capitalism represented maximal freedom and justice, if every able-bodied and able-minded person were perfectly free to live secure from want and misery,
There is not promise of being "perfectly free to live secure from want and misery" against any proviso (honest days work). There are no guarantees, just as in state socialism any gaurantee offered must rely upon the ability and willingness of other people to produce and bear the burden of meeting said promise. So the concept of what 'capitalism' is supposed to mean seems off target.
:If capitalism provided freedom (the freedom to try different methods of providing for oneself)
What does this mean? What different methods - no more than there are currently on the menu and through your effort created for yourself. The rest of the folk are not obliged to supply options to satisfy every person.
: and justice (the justice of having the results of one's labor determined by how intelligently and diligently one worked),
Not quite, but related, it is determined by how useful to self or others your production is. You might work intelligently and diligently at making an umbrella in summer, whilst the next fellow churns out low quality ice lollies and is in greater demand.
: The even playing field that capitalist idealists imagine is an abstraction that does not exist, and perhpas never existed, in reality.
And it never will, so an so is taller so he can reach for the fruit in the tree - and? so and so has friends who get him the fruit - and? It never has been level in the sense you mean, nor would it ever be - under any system.
: In an environment like that, if sufficiently poor, the greatest intelligence and effort will fail, while, if sufficiently rich, the greatest ignorance and vice will succeed. Is that freedom? Is that justice?
This is conjecture. Its reasonable to suggest that a poor genuis has less opportunity to shine than a rich one, but to suggest it is directly linked to wealth, so that upon reaching a certain state of poverty the poor genuis can never shine is silly - unless you mean that state to be actual death. Also the idea that rich means success is dubious. "from shirtsleeves to shirtsleeves" is an expression common in the last century, less so today where old wealth is protected with political pull.
: The socialist does not object to,......
You sound rather different to the 'from each according to ability, to each according to need" socialist. It sounds like you are simply fed up with the machinations and political pull of those called 'the privilaged' rather than against the notion of free trade and justice close to what you describe above.
: Capitalism is like the man who, after having been helped by another man, stabs him in the back and mocks him for being so simple and trusting.
Where on earth did you learn about capitalism? have you read its intellectual fathers like Locke, JS mill, Jefferson, Smith etc? Here you would learn that capitalism and free trade would be represented by two men creating something of value and exchanging it to *mutual* advantage without sacrifice of one to the other.