Those screws are redundant, they aren't supposed to be really tight or it will make it difficult to remove. I believe the screws go right through the hub don't they? Or are they blind screw holes, i can't remember, either way if they go right through, just use a slightly smaller drill bit and drill right through, if you're lucky, the broken stud will catch and drive right out the back, if not, use a pick and carefully try and unscrew them since they should be already loose. If that doesn't work, use the method mentioned above, and use an extractor. Careful not to use too much force since you don't want to break a hardened steel extractor off inside the hole. If all else fails, just replace the hub assembly. You can buy a used unit fairly cheap. Good luck, I hate broken bolts.