I've had a similar issue with that. Leaving out the IOR map(it's often baked even if you didn't paint it) should solve it. Or at least you should modify it with a filter shader or similar.
I haven't tried putting my map inside a Corona Bitmap shader at that time, maybe it could fix the whole thing.
I don't know if Allegorithmic or whoever created the CRN exporter in SP was aware of the color handling differences between MAX and C4D.
edit: The simplest solution would be to have the metal maps in one material and the dielectric in the other, one of the 3 presets in SP gives you a blend mask which you can use in the layer material.
If that doesn't work, then I'd need to have a look at your SP layers/materials and output setup.
Btw, the reason why the worn edges are rendered in black is that they're usually used in a metal material, Corona only uses a dielectric one. If you have a rough metal map, it would look different on a metal material. So the best way is to either fake it with the layer shader or simpler, the layer material.
There are some approaches to refl. maps using the diffuse channel, but I would stay away from that, as long as you don't need it.