About the probability, you must have mutation in all cells. All must be a positive mutation. So, that is the probability of a successful mutation as stated in Scientific Creatinism.
Well the mutation happens in the DNA, which is what builds the cells. Think of it like a recipe. If I add chocolate to a cake, I have 1 cake with chocolate. If I add chocolate to the recipe for a cake, every cake will have chocolate. It's the same here.
The DNA creates the cells. Every cell in your body contains the same DNA, but not all of the DNA is used to create every cell.