1. Does God know everything?
2. I cannot argue that Wesley believed Calvinism to be right theologically, but he came to believe that his friend, George Whitfield, was not a plague. In fact, he considered Whitfield to be a blessing.
As with so many things why He did this will have remain a mystery. Just as it is a mystery to me why Christ had to die for the sins of people God has already selected for eternal worship.