As a layman I don't know what consensus geologists have about your questions. But I think the answer to the last of your questions would be yes. The colder/soli

As a layman I don't know what consensus geologists have about your questions. But I think the answer to the last of your questions would be yes. The colder/solid slab is showing up in data gathered about seismic wave propagation speed. I have the model in mind of an ice cube in water, except with higher viscosity and of course more heat. There would be a penumbra of cold water surrounding the melting cube until a point where there would just be cool water, vaguely cube-shaped. Plus the motion would smear the penumbra but not the remaining solid. Also my guess is there aren't a lot of precise measurements, so the usual blur from that. Like an ice cube in water, if you test the solidity of the cube you find that the cube hasn't softened, just shrunk. I looked at a few of the plots but only the Cocos plot clearly suggested a 'slab' to my eyes, and probably for the wrong reason: The plot looks like it is divided in half by the blue area. Looking more carefully and reading, I think what I'm seeing is two depths of the same object. The plate subducting under the West coast of the Americas is apparently appearing as an anomaly or slab at two depths.