Yep, Sean is right on the money, if something is flat and it needs to be rounded, round it.
Best part about a backplate is it can be customized, including softening the edges and radius of edges and cutouts. I might recommend starting with some strips of emory cloth rather than a file, though same idea. You could cut strips of emory cloth off a full sheet to various widths to get in the corner as needed, you could get them wet to help remove the stainless dust as you're sanding., You'll probably blow through cloth working with stainless, its a pretty hard material. I'd make 12"-14" inch strips about an inch wide and pull them back and forth over the edge of the top of the plate until its nice and contoured starting with like a 200 grit, working through grits to as high as you want to go for the desired gleam.
Maybe another thing worth considering - are you hanging weight pockets off the waist belt that aren't also bolted to the backplate? If you have trim pockets hanging unsupported off the waist belt, I could see that weight and friction causing some premature wear. Not much that can be done about the shoulder wear though other than softening up the radius of the top and cutouts and making sure there's no edges to wear on the harness.
On the shoulder straps - are you getting the wear where the harness goes through the slot back toward the rear, or where it comes over the top edge of the plate?