Years ago I decided that the issue of "free will vs. determinism" is irrelevant to questions of ethics, and untestable with respect to matters of science. Since then I have tried to avoid wasting time on it. But it comes up every now and then in Freethinker circles, and many people are lured into arguing at length over it.
Our ordinary practice is to ascribe "free will" to beings which are conscious and intelligent. "Conscious" meaning that they have an internal ("mental") model of the external world, which they use to anticipate the consequences of different "imagined" courses of action. "Intelligent" meaning that their model is complex and sophisticated, and their imagination likewise, so they can find courses of action that will serve their purposes even in novel situations. "Free will" in such cases means that the great bulk of the IMMEDIATE causes of their actions lie inside their "skin" rather than outside, AND that their actions are not easily or reliably predictable by an outside observer.
This use of the term "free will" does not require denying the hypothesis of "universal causation", nor does it depend in any way on whether "causation" is always a single-valued function (i.e. whether the same inputs always produce the same output, or whether instead the output may be any of several values with some statistical probability for each.) In other words, this use of the term "free will" is fully compatible with "determinism". Beings with "minds" sufficiently sophisticated to have "free will" may operate their "minds" deterministically.
We assign "moral responsibility" to beings with "free will", we assign praise and blame, rewards and punishments, to such beings, because that is the easiest (often the only) way we know to intervene in the causal chain. We want them to behave in one way rather than another, so we initiate some causes that we hope will have the effect of modifying their behavior. We hope they will include in their "mental" model that we will respond to their actions with praise/blame, reward/penalty, and that they will therefore "choose" a different course of action. The hypothesis of "universal causation" is irrelevant to this.
If we gain some ACTUAL KNOWLEDGE of the causal chain affecting their actions, then we may intervene at a different place. For example, if we find that childhood exposure to high levels of lead in the environment leads to neurological damage that results in a lack of ability to control impulses, i.e. their ability to control their own behavior by "rationality" is impaired, then we may seek to reduce crime by banning leaded gasoline, lead-based paint, lead solder in water pipes, and so forth. But this is not the same as "determinism", considered as a philosophical hypothesis.
"Determinism", the hypothesis of Universal Causation, says that "all events have causes; there are no uncaused events". This is a universal claim. The critic may offer as a counterexample some event with no apparent cause. The believer in Determinism will reply "the cause may be unknown at present, but there must be one". This is not something that can ever be proved or disproved, by any amount of evidence, short of complete examination of the entire Universe throughout all of Time. It is a starting assumption, a working hypothesis. Some have claimed that it is a NECESSARY assumption for the practice of science, but I don't think so. Science can be practiced perfectly well under the assumption that many/most events have causes.
So: I see no reason to spend one more second debating the question of "free will versus determinism".