Regarding the hole in the middle of a fusing slab. I have learned that this is more common with 96 coe glass, used on top of kiln wash. It will happen to any glass but 96 is usually the cuprit. It is an air bubble that grew until it burst, this is usually caused by the glass sticking just a bit to the shelf, if I understood correctly. The problem is easily solved by using shelf paper instead of kiln wash.