I don't think modality and determinism are in tension -- this is a misunderstanding behind the free will debate.

Imv, it is literally true that "you could have done otherwise" without there being a violation of determinism.

How? Determinism, in this narrow sense at least, is about how events in the physical world relate across time, ie., that necessarily P(later|earlier) = 1. These events are to be thought of as infinitely precise states of the maximally basic stuff of reality, ie., all the info that possibly exists.

But these arent mechanisms, these are states. As soon as you describe relationships between states, ie., causal mechanisms, you're talking about what would happen if the universe were in some state that it may never enter.

I take it to be a basic property of reality that these mechanisms are (at least as) basic as these states. Eg., that the motion "of the most basic stuff" is as fundamental as "where that stuff is".

So, eg., suppose there's a basic atom A and basic atom B and they move this way: A repels A, A attracts B, B repels AB, B attracts BA, etc.....

Now this behaviour is a basic part of their existence: were there to be a universe of AAABBB, then "this would happen"; if BBBBBAAA then "something else would happen".

The "initial conditions" of the universe, ie., it's state prevents its mechanisms from ever entering certain states. But those states are possible given those mechanisms. It's in part what "mechanism" means that it is possible to enter more states than just which ones happen to occur.

So, on free will, what does it mean to say "I could have done otherwise" -- it means that the relevant causal mechanisms make genuinely possible many states. (But the actual initial conditions precluded observing more than one).

Or as a layman would put it: you would have been kind were you a different person; so, your cutely was determined by the kind of person you are. It is because of who you are (state) that what you did (free causal mechanism) was cruel (particular state obtained by operation of causal mechanism).

This may make it clear what people mean when they say, "well your brother wasn't cruel!" as if that mattered. Well: it does matter! It shows that the causal mechanisms we call, "people acting in the world" are so wide open (free) that kindness is possible.

Thus we do have free will. We are free insofar as we are in motion: our possible behaviour is much greater than our actuality. And we are determined: we, by bad luck, arent in a world were our better behaviours are realised.

It is a fundamental property of those particles above (A, B) that were they alone they wouldnt move. You cannot eliminate that property for talk of what they happened to do in the actual world. In the actual world they are really freer than can ever be observed directly.

But it is trivial to observe this indirectly: we can pull A far way from B and see what happens (etc.).

Likewise when people say "You (qua causal agent) could have done otherwise, and you didn't, so you're guilty!" there is no error here at all.

You were guilty precisely because your actions were not accidental, were not indeterminate or random. Your actions were determined by your state. And we judge that state to be one of guilt for a crime: possessing some intent and means to kill, say.

It is in this way determinism is required for free will; and required for a modal universe. If the motions of particles were indeterminate they wouldnt be causal.