The past is merely the record of what used to be the present, and the future is only what will be the present when time unfolds to that point. It is the present where the action is and, essentially, is what produces either the past or the future. So the present is what is necessary for either the future or the past to have what existence they might, and never vice versa.
There are of course, circumstances which were in motion in the past and which determine the future to some degree. A huge asteroid could be hurtling toward us from ages past that will impact our future in very tangible ways. Celestial bodies move according to the laws of physics and their courses can be charted and accurately projected. Earthly bodies, and by that I mean humans, cannot--their responses to their environment, to one another, to God, or to themselves cannot be charted because their free actions are uncertain until taken (the Heisenberg principle of free agents?).
Therefore the future cannot be fixed in any real sense, because it is dependent upon a present which is in flux. A future which is dependent upon that which is in flux has to be in flux itself. It seems to me that if there is any truth at all to the notions of will, freedom and spirit (which produce flux) it is impossible for the future to be fixed. Nothing is actually written until it's written in its present.
That does not mean the future cannot be known.
If an observer could view all of the presents that will ever exist from an observation platform outside of time, the future would be known to him, exhaustively, by observation. I see no other possible way than this for the future to be known and freedom for agents to exist in the present (see this). If that observer were infinitely wise and powerful, he could shape that panorama of time by an interposition here and there (or as often as he saw fit) without affecting the existence of freedom in any present. In being able to do so, I see no reason to posit that such a being would need a mental "trial run" (i.e middle knowledge or deterministic decrees) in order to do so.
The future, whatever it is, is made only in the present. That the future can be known by a timeless God does not make it fixed, for if some other reality obtained due to freedom in some present, that is the future that would be known by God from observation. For the future to be fixed, will and freedom would have to cease to exist in any true fashion. I see no biblical warrant to suggest such a course, and to posit such is to misunderstand the nature of the future and put the cart before the horse.