Non-exclusive use of farmland or wasteland (or even - at least until maintenance complexity is considered - offshore) isn't a major barrier; we have many many miles of wires and pylons crossing farmland for regular terrestrial grids already. A rectenna looks more like a mesh than a set of panels: the idea is that the visible light spectrum passes through and the target microwave frequencies don't, so unlike a photovoltaic solar farm the land underneath is fine for farming. Nominally the land is safe to be occupied or passed through by humans too, although they might take some convincing. It becomes a problem when people start trying to get the rectennae banned...
Never. As the post you apparently quoted without reading said, we're talking about a rectenna composed of pylons and wires structured as a mesh that doesn't interrupt farming and can theoretically be offshore, not panels here
Space based solar power has much bigger challenges in gaining acceptance, proving the physics works in real world conditions and delivering on its promises than the theoretical possibility that there is absolutely no unoccupied region of land or sea