Of course, God planned for the possibility of sin. But that doesn’t mean that was part of the plan, but a deviation from His plan. Gods original plan was for no sin and no Islam. That was The Plan.
Not being a predestinationist, I agree. I do believe that for every plan, God has a backup plan.
I wonder if islam would exist if Ishmael had not been conceived? Kinda moot, I guess.