Often the bolts would loosen and fall out, but sometimes they would break. They seem to always break if you are trying to remove them! If they just fell out, I would clean up the threads with some rust penetrant and a 6Mx1.0 thread chaser. Then, I would get some slightly longer 6Mx1.0 bolts and flat washers (as previously suggested) to secure the shield. If you can't, then it'll be okay with removing the heat shield. It'll be fine and won't be the first one. I wouldn't be worried about the sensor.