If a farmer can't buy the amount of grain he produces
That's not the premise at all. This isn't about the ability to extract a 1-1 ratio. It's not about extraction at all. It is posing that the wage earner is not free despite the value they produce.
They're producing products that increase that value. Grain into flour, flour into bread. Each phase of that labor is commodified around a wage which does not increase in value based on their produced value. Those wages earned are a portion of extracted value.
The wage earner that produced the machinery was not paid a wage based on the value of grain produced. It's something different. And that difference is where the wage earner is not free.
To your credit you have posed scenarios where the farmers and laborers are also the owners of the operation, which is a big piece of this puzzle. But something of note:
Same goes for whoever owns the land
What is the productive labor that is ownership of land? What value has the landowner produced in the bread?