the government would treat valid visas and green cards as a sort of contract, and that they would have an obligation at least to continue to honour it once issued.
Contracts depend on the negotiated terms, if someone applied for a visa that could be revoked at any time then that's what can happen, even if they cry
From what I've seen first-hand, the answer is no
what you've seen are low wages suppressed by migrant labor, once they are gone you can say you have some experience
in fact our economies depend on this disparity continuing
doubt it, since farmers only get a small fraction of what the product sells for that means that labor is a tiny fraction; excessive and ill-gotten profit is what is at stake
After a number of decimal places, nobody gives a damn.