As to your other points, that's what the Soviets said when they created the farm collectives. It will be better for everyone.
The actual takings clause does appear right there in Amendment 5 in the Constitution. “private property [shall not] be taken for public use, without just compensation.” Public use and just compensation are both subjective issues open to interpretation. Your mother would have the right to fight the position in a civil action, but it is entirely subjective as to what the courts will decide is both public use and just compensation.