Malcolm is certainly not aware that he is the one who makes the fulfillment of the witches' prophecy possible. It is him who confirms Macbeth's worst fear. Malcolm's plan that soldiers should carry a branch from Birnam Wood so that Macbeth is unable to evaluate the exact number of soldiers turns out to be fatal. This is because the witches told Macbeth that he has nothing to fear until Birnam Wood "comes" to him. When Macbeth learns from the messenger that "the wood began to move," he is astonished but vows to fight back.
I do not think that Malcolm is the " unconscious agent of destiny" because this could imply that the witches are the conscious agents of God and destiny and that whatever they prophesy will come true. Rather, Macbeth's tragedy lies in the fact that he is so blinded by the witches' prophecy that he is unable to think clearly or even question their so-called powers. Whether Malcolm's strategy is a matter of coincidence or the result of King Edward's "secret" gift ("he hath a heavenly gift of prophecy...") or something else, Macbeth is solely the one to blame for his actions and consequences arising from them.