Apologies, roulis,
Yes, I misread your 15% slope as 15 degrees. (Maybe I need new reading spectacles!) Must admit I am not accustomed to seeing descent profiles or glide-slopes expressed as percentages... Climb gradients always have been, of course.
As already stated, I am not commenting here on the ergonomics of the A320 FCU, but do not disagree with your last paragraph. Having flown similar approaches on the type before and after the modifications, I'm broadly familiar with the accident scenario and the main hypothesis.