Salaam o Rahmat,
I once read your post that you wrote Du'a even might bring about extrinsic changes.
Do you believe a Dua for killing a person by a criminal might cause the death of that person, or you believe the maximum effect of Du'a can be showing the way of killing to the criminal only?
I think if we consider the maximum effect of Du'a more than showing the way to reaching the desired thing (i.e. Du'a could do/cause a work/happening for us directly such as causing the death of person by a Dua), then we have accepted magic and other superstitions.