Wow..........just wow!! Shouldn't she be on her honeymoon or celebrating with her new husband, to waste the first few days of her marriage obsessing with her cake is ridiculous.
I've done this for a long time and I'd like to share a useful tip with all of you guys for the future. I always carve away the crust on all of my larger tiers (white or yellow cake). So when the cake is sliced they just see the same uniform color from middle to edge.
From a customers perspective I can see why she thought the cake was darker on the edges they CERTAINLY WERE NOT BURNED, I'm not saying it was burned so please don't flame me.
When you slice off the edges i.e. sides, top and bottom of a layer, you don't get any dark color at all.
As the OP said it's hard to not get the darker edges (corners) whatever, when we're baking the larger layers, but you can get rid of them.
I seriously doubt this wrench will contact an attorney, she's calling your bluff.
Sorry this happened to you and please don't take my advice as siding with this lunatic bride because I most definitely am NOT.