Not necessary, because alfalfa is just one example of a “green-manure” crop. But other “green-manure crops” could be grown instead. Also, the author doesn’t need to assume that “most” farmers will grow crops. As long as enough farmers grow the crops to improve soil structure, the reasoning still works.

Not necessary, because even if chemical fertilizers have a positive effect on “green-manure” crop growth, it could still be necessary to abandon their use in order to get farmers to grow “green-manure” crops again.

The argument concerns what’s required to improve soil structure. Whether soil structure influences other things, such as soil quality, is not relevant to the argument. And even if it were, there’s no reason the author must assume that soil structure is the “most” important factor in soil quality.

Not necessary, because the effect of chemical fertilizers was to cause farmers to abandon “green-manure” crops. Even if the chemical fertilizers do not directly destroy the soil structure, they still had the effect of causing abandonment of the practice that rejuvenated soil structure.

Necessary, because if it were not true — if many farmers in the region WILL grow “green-manure” crops even if they keep using chemical fertilizers — then there’s no reason farmers would need to stop using chemical fertilizers to improve the soil structure. They could keep using the fertilizers and start growing the crops again.