The clip-in nuts although great when new, are a nightmare when seized. Once seized the square part of the nut will round in no time as it's so shallow and then the only other thing that can prevent the nut from turning is the bumper itself, and in most cases the plastic will give up first and the nut will start to cut into the bumper and break through if you continue.
Another simpler, cheaper option would be to use some slightly longer bolts with large washers and nylocs. (Stainless would obviously be the better option). As long as they're done up tight enough to pinch the too together tightly, but not too tight that it crushes the plastic (the reason for the shoulder on the original bolts) you'll be fine
