Re: screws: my impression is that the screw(s) come loose from the plastic nut, the nut falls away so the screw now bears the weight unequally, tearing the hole in pannier material just enough to make the combo ineffectual. I now tighten the screws more regularly (and now know they do get loose) and put little metal backing plates on to help distribute the stresses on the screw.
As far as Ortlieb warranties, we bought them at MEC so swapping them was easy (props to MEC). Also, we are in the midst of our tour so when told about timeline to await developments from Ortlieb, we chalked it up to lessons learned.
ps My Arkels have been great