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How to Composite the Roughness Channel in Photoshop:
I have been unable to find out how to composite the roughness channel in photoshop. Is anyone able to tell me where in the layer stack does this channel output go, what is the blending mode and does one add an adjustment layer to it, and if so which adjustment(s) are commonly used?

It's a channel for masking purposes when compositing. You shouldn't expect to reconstruct the surface roughness in the post.