I think the difficulty in communicating the idea of free will in a rationally sound way comes from the fact that, as Schopenhauer mentions, while Will and Idea are both different perspectives of the same thing, the fact that they are different perspectives means that there is a mutual exclusivity between them. You can't communicate the notion of Will through Idea, and the Will cannot be guided by Idea, yet the two are one, in that each is always the other. Yet there retains a strange distinction between them, in which each has a sort of world of its own.
Suppose, as an example, we treat a human simply as an expression of a particular set of genes. Then, from this perspective, the whole of human activity is reduced to the propagation of the various genes. But on the other level, the question of the genes appears unimportant, and it is completely the human sphere as experienced by the individual which is our focus. And extending this even further, we can regard the human being itself as only a part in the larger system of culture, and view the flow of events as the flow of culture. Each of these three examples is equivalent in the sense that they are describing the "same thing", yet they retain a strange sort of independence from each other.
So, the same is the case in the question of Idea vs. Will. Inter-subjective communication occurs through a process of a) Idea transmission, and b) enforcement of Will. If we look at a), what we have is agents who take in a set of information, and deal with this information in some particular way, output some other information - a pure, detached, dealing with Ideas. This is how we typically think of communication - as a pre-determined algorithm, free from any notions of enforcement of Will. Yet, on the other hand, we may view communication completely in terms of the Will - basically the view which Nietzsche proposed, in which all logic is arbitrary, and what is actually going on is a continuous battle of the Will of the various Subjects, from which the seeming order which Ideas deal with is an unimportant by-product. Due to the nature of this view as distinct from Idea, it cannot be communicated through Idea, and so any attempt at doing so will prove futile. Rather, what we are forced to do, is to make this notion of Will known by enforcing our Will on the world. We apply a force to this object, to this person, to this emotional state - but we cannot Will an Idea.
Thus we have seen the source of the notion of determinism. Determinism seems so irrefutably true because it is an Idea, and it is communicated through Idea, and it addresses Idea, and Idea is totally free from the notion of Will - determinism is always the more logically sound position, since logic is a deterministic system. Yet, the relation between Will and Idea remains, and we see this Idea as having an effect - or at least, indicating an effect - in the realm of the Will. When determinism is prevalent, the most illuminating picture of the world will be that of Idea, for a view is being propagated which benefits Idea whilst harming the Will. When, on the other hand, even the realm of Idea is suggesting an importance of Will, we see a richer picture when we turn our view to the realm of the Will, for when this is the case Idea is essentially self-destructively shifting focus over to the Will.
We see this for example in the cycling of societies. When the picture of the realm of Will becomes less important, which we may possibly represent with thermodynamic models of reducing entropy of power, the realm of Idea becomes increasingly emphasised. And by exactly the same measure, as the realm of Idea is increasingly focused on, the realm of Will suffers more and more to the point that it becomes meaningless. Similarly, as Will becomes the focus, Idea declines. As Idea declines, Will prospers. 1
What happens, then when a culture who emphasises Idea meets one who emphasises Will? Presumably, one spreads its Ideas over to the other, the other spreads its Will, until equilibrium is obtained.
1 - This notion that that which is beneficial to the Will is destructive to Idea will need to be investigated. Indeed, there will likely be certain meta-characteristics of the system which cause both to be heightened or lessened in equal proportions, while the two simultaneously, and orthogonally, exchange focus.