Man, the scene and these breakdowns are incredibly well done! 
Really love it! I would have one correction though^^ When you describe that your workflow for the shield an dustlayer is not 100% PBR, I have to disagee. Its actually quite PBR to do it like that! To push it, you could also add a super fine grain to it. But in general, you would use something like a grain mask for metal to say dust is 100% black (nonmetal) but since the rez of your texture must be super high to support this, doing what you did is absolutely right, since the grain should be almost too small for the human eye to see it over like 1m distance, so it just blends 
But anyways, very good work!
Cheers