You've got gravity motivating the descent, obstacles causing resistance shaping the path, and the distortion of the graph like the bowling ball on the bed distorting space-time. Perhaps we could make this a little simpler?
Why not go with a standard description of choosing that everyone can understand:
(1) We encounter a problem or issue that requires us to make a decision before we can continue. Perhaps we've just been seated in a restaurant and we've picked up the menu. Unless we make a choice, we will go hungry tonight. The waiter assures us that we can choose any item on the menu. The chef has the ingredients to prepare any meal we order. We see several options that we like. Each of these options is a real possibility. And we can choose any one of them.
(2) We consider (a) the Steak dinner. Is it likely to satisfy our hunger and our tastes? Is it consistent with our dietary goals? Now we apply the same criteria to (b) the Lobster dinner, and finally to (c) the Chef's Salad.
(3) After these considerations, the Chef's salad seems better to us than the others. Our thoughts and feelings confirm that it is one we want the most.
(4) Our will, our specific intent for the immediate future, is now set.
(5) This intention then motivates and directs our subsequent actions: "I
will have the Chef's Salad, please", we say to the waiter.
(6) After the meal, the waiter brings us the bill for the Chef's Salad, holding us responsible for ordering the Chef's Salad.
Now, just to avoid confusion, each of these events, (1) through (6), was causally necessary/inevitable from any prior point in eternity. We could easily put "It was causally necessary that ... " as a lead-in to every one of those events. In fact, assuming a world of perfectly reliable cause and effect, we could always put that phrase in front of every event we describe. But why bother?
Universal causal necessity/inevitability never changes anything, not even free will. It is a background constant, as if it were on both sides of every equation and can be subtracted from both sides without affecting the result.
Except it doesn't actually have to be true that you can choose B to have chosen A.
I could not have "chosen" A without there being another real option. So, logically, it has to be true that I can choose B, in order to get to the next step, evaluation. I can choose A must be true. I can choose B must be true. This is the way "can" functions within the choosing operation. It is logically impossible to choose between a single possibility. For example:
Waiter (a hard determinist): "What will you have for dinner tonight, sir?"
Customer (hungry): "I don't know, what are my possibilities tonight?"
Waiter: "Sir, because we live in a deterministic universe, there can only be one possibility".
Customer (disappointed): "Oh. Okay. Then what is that possibility?"
Waiter (angry): "How would I know? I can't read your mind!"
We have evolved certain concepts to deal with uncertainty. When we do not know what
will happen, we imagine what
can happen, to prepare for what
does happen.
The meaning of words is derived from their practical function. What we "can" do, we may do, or we may never do, we don't know yet. And that's the uncertainty that we must resolve with the choosing operation. At the end of choosing we have certainty. We know what we will do. And we also know for certain what we could have done, but didn't do. The "could have done" is just as certain as the "will do".
You suggest that the graph needs to "self-modify". Well, empirically speaking, the brain is in a continuous process of self-modification. And each of these modifications will be causally necessary/inevitable from any prior point in time.
Free will is a deterministic event. There is perfectly reliable causation leading up to the point where we are faced with an issue that requires us to make a decision. Within the choosing operation each step is also reliably caused by prior mental events. The choosing operation is where we exercise executive control, because the choice determines our actions. Our actions reliably cause what comes next (our agency). And perfectly reliable causation continues as events caused by our actions reliably follow. As the song says, "May the chain be unbroken", and it is.