I think this is the crux of the matter, or at least, this is where I can't follow your logic anymore.
My problem is this:
In what sense can one be said to be able to change ones mind if one won't change it under any thinkable circumstance whatsoever? What does that even mean? How would one go about testing such a thing, even with the cooperation of the omniscient being in question?
The problem is that an omniscient being cannot be understood by us sufficiently, to deal with the issue of freewill. If you examine a sequence of choices in any detail you come up against potentially infinite regression. This may be manageable if we're dealing with numbers or atoms. But when we are dealing with a
being and what this being
knows it becomes impossible in any relevant way.
I am only required to remind one of the implications of infinity when applied to knowing, to illustrate this.
Say I was deciding what to cook for dinner in a hypermarket. The omniscient being would
know/be aware of an infinite variation in possible outcomes in the hypermarket. Indeed with such insight as one would expect from such a being, the being would know how the combination of every atom in an infinitely large universe would affect the movement and interaction of every atom down to the Planck scale (and infinitely beyond it). In fact it would be aware of many infinitely short segments of time between the change of state between two atoms in the hypermarket and its implications.
When the choice was made about what the dinner would consist of, there would potentially be infinite variation in influencing factors for the precise chemical change which swung the issue to become lost in. Such complexity would be a breeze for this being to know. In fact it would know infinitely more details than that.
The distinction between a free choice and a pre-determined choice would become lost in the detail and the difference in cosmic impact between the few choices I was actually realistically likely to chose from might be so slight that there may be space for a genuinely
free choice to be made.
Not to mention the large number of
choices I was involved in between the entrance of the hypermarket and the point where the menu for the evening were known to myself.
And during all this I knew I was either going to have omelette or curry. I just hadn't made up my mind yet.