Yes, like you said before, this goes back to who has the burden of proof. You say that because we experience ourselves as possessing free will the burden is on those who question this to prove its absence, buy I think there's something to be said for the POV that the burden is on the defender of free will to define what they mean by it - and I think that's not as easy as it might seem.

Yes, agree that it's an activity of the body or an emergent property that arises from a certain way in which the body acts. But I think this too is very tricky: for example, say we wrote a computer program that behaved in certain ways depending on the inputs it receives - not an unusal case. But say we made this so complex, its reaction dependent on so many different factors and then cnaging in response to the relationships between those factors, and also on its own past behaviour, etc, etc, that we have no way of predicting how it will act in any given situation except by simply running the program and seeing what it does. Must we now say that our program has, if not free will (since it isn't necessarily intelligent), then some other kind of freedom to behave?

This of course has real-world implications in the field of AI: at what point do we get to say we've created a real artificial intelligence? Is it a matter of complexity or is something else involved, and if so what would that something else be? And would such an intelligence, once created, necessarily also have what we call free will?

Well I agree that we don't live in the clockwork universe imagined by 19th-century physicists. But I'm not sure we need to reduce the question to an either/or of determinsim vs free will. Some philosophers would question whether the concept of free will really means anything outside our subjective experience.