The past is merely the record of what used to be the present, and the future can only be what will be the present when the present gets to that point. It is the present where the action is and it is that action that 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?).
Freedom is entirely wrapped up in the present. It exists in the moment of decision. Insofar as choice is concerned, the past is inert and the future is is inaccessible. Decisions are made in the moment their action occurs, necessarily. Some choices are made which effect the future and/or are influenced by the past, but freedom to act, to choose, always and only happens in the present.
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, and by that fixity control the present. Nothing is actually written until it's written in its present.
That does not mean the future cannot be known.
If an omnitemporal observer (God) could view all of the presents that will ever exist from a vantage outside of time, the future would be known to him, exhaustively, through observation of the present. That the future is known by this God would not make it fixed, in the sense that it determined the present, for it is the present in which action occurs, the uncertain becomes the established, and which God observes and thereby knows the future. God knows the future because God has seen the present timelessly, but it is never his knowing that causes what he sees.
It is, in fact, a confounding of cause and effect to assume God's knowing the future would bind the freedom of an agent in the present. That God knows an agent will act in some fashion at a particular time is not equivalent to saying that the agent must act in that fashion at that particular time because God knows that agent will. On omnitemporal observation of freewill, the act of the agent causes the knowledge, the knowledge doesn't cause the act. Seeing timelessly is out of our wheelhouse as humans, and therefore justifiably confusing, but it should be straightforward enough to perceive that seeing an act omnitemporally cannot be said to necessitate causing that act in time.
Furthermore, an infinitely wise and powerful God could shape the panorama of time by a directive interposition here and there (or as often as he saw fit) without affecting the existence of freedom, generally, in any present. He could shepherd time to an appointed end without meticulously determining anything that occurred in time. In being able to do so, I see no reason to posit that he would require a mental "trial run" (i.e middle knowledge and/or deterministic decrees) in order to do so. He saw all at once, once he created.
I see no other possible way than this for the future to be known exhaustively by God, for creaturely freedom to exist in the present (see this and this), and for God to not have conceived evil before evil existed. We'll address that last concept later, for now, suffice it to say that if the future is fixed and thereby determinative of the present, will and freedom would have to cease to exist in any meaningful fashion. I see no biblical warrant to suggest such a course. To posit such is to put the cart before the horse and totally miss how God knows the future.