Sometimes, other times it's just wind farms are put in by companies who do wind farms and buy those rights, often from regular farmers, who continue to farm around the turbines. If you put in solar, then there's nothing left to farm. Though for grain crops solar is likely going to be more profitable on an acreage basis.
That's definitely part of it, but as solar panel prices continue to fall I suspect that the marginal cost of adding on solar panels to a wind installation will be low enough to justify it even in sub-optimal sunlight conditions. You've already got the land and the power transmission infrastructure taken care of, so it's just the additional cost of the panels. It could start making sense.
Exactly, anywhere plants grow you can put solar, Germany has one of the highest concentrations of solar but pretty dismal sun. Germany stopped building Neuclear Plants and didn't like buying Natural Gas from Russia, so they basically invented the modern Renewable industry.