Your bolts should be 17mm not 18mm maybe that's why you have rounded two of them,you can get extractors that fit over the head of the rounded bolts and as you undo it bites into the head and undoes it,once the two rounded bolts are out you should be able to remove the wheel and work on getting the remains of the sheared bolt out ,or you could try welding a nut on if you got a decent welder to use,ive had to do this many times.