I would think sanding it down from the inside of the face plate should do the trick if it doesn't interfere with the magnet.
The problem is the top of the face plates don't flush right up to the front side, there's a slight curve where they meet, that's the edge that keeps showing. You'd have to shave it off or sand it, and then risk it not fitting into the slot properly. It's just an issue for photos really, on shelf display, you can't tell.
Yeah, you described that perfectly.
Like you said, you'd have to sand that whole curve off the top so that it was a sharp edge, but I don't think it would work. I think you'd just end up with a gap between the face plate and the cowl edge. :/