I believe you define free will as a supernatural ability - the ability to make decisions that ignore causality. I don't define it that way. I define it as the ability to make a decision between multiple options (as opposed to being constrained so there is only one option). Whether or not I would make the same choice if someone rewound the universe is irrelevant, since it's pure speculation that nobody can possibly test. What matters is whether the result of the decision was preordained or not.
I enirely agree that we can't rewind time. But the point of my "save game universe" thought experiment has always been this:
The situation is that at a particular moment in time, you have a complete, universe-sized set of variables. On the personal front, you will have all your memories, all your preferences, the thought that last was in your brain. Your preferences will be at a particular state, your bodily functions, your needs, your physiology. In your immediate environment, you will a specifc level of light, heat, gravity, odour, music, and so forth. In a particular situation, all the variables are - at that moment - fixed. You could say there are no "variables", just specifics.
The solely causal view of the universe is that in a particular set of circumstances, a particular thing will happen. All the physical laws that exist say that state A, with input B, will inevitably lead to output C. In an entirely deterministic universe, that would be the case. Free will would not exist (though the illusion of it might), it could NOT exist. A leads to B leads to C, with no opportunity for deviation.
The wrinkle is that there are quantum-type events that add miniscule random factors - a wuantum blip in a neurone firing would lead to a slightly different result - but, crucially, an identical blip would lead to an identical result in the same neuron. So because of tiny random factors, the result will not be predictable - but is still deterministic once the random effect becomes defined. Free wil still does not exist (though again, the illusion of it does, and more justifiably), but still there is nothing chosen
there. The result switched from being determined, to being random.
The free will that is being proposed in this situation is that there is some mechanism by which a very specific set of circumstances can be in some way lead to two or more non-random outcomes. And that's the bit I just can't grok.
Even moving to the metaphysical level, I can't grasp it. Even if there is a soul, or a mind, or whatever - some non-causal thing that can in some way make a genuine choice as to how to direct a thought or action, so far as I can tell that simply leads to many more problems than it solves. How exactly does this mind make those decisions? That mind has the same data to work from, it has the same preferences, it has the same history and character.....what is it about that mind that enables it to make a genuine non-random choice? How does that work? That's the part that I always get stuck on when we hit free will - how that non-random decision can come from a defined set of circumstances.
It's why I do not believe free will exists, in the sence that anyone can REALLY choose what they will do next. But to tie it back to the OP, there is still a point to arguing with the religious: every time we interact, we change their variables slightly. We add to their memories, we alter their environment a little. They will never again experience the exact same set of circumstances, but the next time they are in a similar environment, their internal state will be slightly different - so their (determined) outcome at that point will be slightly different to what it would have been if we had NOT interacted with them.
Of course, the glaring elephant in the room is that without free will, we do not - CAN not - "choose" to interact with them in the first place! The illusion of free will is so persistent that even though I am convinced of its non-existence, I still still mostly operate as if it were real.
But what I will say is this: when I am able to stop and consider, and examine the world, and look at the "choices" made by the people in it (from the trivial to the huge).....viewing those choices as the inevitable result of their environment and history makes them a whole lot more understandable than viewing them as genuine choices, and trying to understand why people would make such a free choice in those circumstances. I realise that that is not in any way a conclusive proof, but I have found that the non-free-will hypothesis seems to answer a whole lot more questions about the world than the free-will hypothesis does.
Not least: why this debate keeps on keeping on with the same usual suspects each time!!