Jesus and free will

The point of prophecy in the OT was often to get the Israelites to change their behavior.  It was God's way of saying that bad things will happen if you don't move in a new direction.  It wasn't always a unilateral "this will happen no matter what" statement (though it was often phrased as such).

As it relates to your OP, Jesus was submitting to the will of the Father.  But he certainly had the authority to call on the legions of angels.  However, he knew what the plan was, so he went along with it.