1) As far as I can tell, the metallic parameter controls how the primary colour is used i.e. whether it goes into diffuse (metallic == 0) or into specular (metallic == 1) or a mix somewhere in between. When metallic == 1, the diffuse colour is black (0), when metallic == 0, the specular colour is assumed to be 0.04 (which is typical for the majority of non-metals as they tend to lie between 0.02 and 0.05 for common things). Perhaps something similar to this in shader terms:
2) I *think* the cavity map is multiplied by the resulting specular value to help darken it - light from an environment map tends to look a little too bright in small creases without proper occlusion being taken into account, so this is a reasonable work around for it. They no longer need a separate specular input (except for special cases) as it's handled by the single colour input and the metallic parameter.