All those bar Shadow/Diffuse are additive. for the others you can propably just pull the alpha from the beauty pass and use that. out of interest why are you splitting it up so much? beauty, shadow and bg would be plenty
a note on the clipping. The shader on the occluders is totally irrelevant in this case. you dont let them render, only cast shadows. I usually place them just below the hit point. so that the 3d element gets the shaowing but doesnt get a tell tale ring aound the intersection point (AO) Also mentalrays Matte shadow shader…