Are there any kind of workflow scripts that automatically import and arrange pbr materials (let's say pbr materials from cc0textures) into the new physicalmaterial from corona ?
Major PBR texture providers like Megascans, Poliigon and others, have specialized exporters. Also you can find various PBR scripts on scriptspot and in other places. In other words, yes, such tools do exist.
Can you (it seems not) use a layered mtl with different displacement maps set up in the sub materials ? It seems only the basemtl has the displacement visible, and not the ones from the layered ones. (Would be usefull to mix several materials that have different displacement maps)
As you already discovered, displacement only works in base layer of layered material, but that has nothing to do with physical material, it's the same for legacy material as well.