it looks like the fact that the lower mounts push the rack so far rearward that the weight of the pannier is being allowed to be farther forward than is normal (heck with my large feet, I have to hang the pannier on the very back portion of the rack) this would put more weight forward causing the rack to rotate torwards that front, bending the support. Until you can fabricate a more robust mount, try moving the pannier more toward the rear so the weight is pushing down on the back of the rack, instead of on the front of the rack. You want the support to be under tension, not compression.