Free will is relative to humans and we define it by what we do. What we do, as has been stated repeatedly, is based on electro-chemical interactions and is a product of science. That may mean it's not very "free" but is still a pattern of how we manifest and does exist.

This would work on first sight.

But I have my doubts that these multiverses would be deterministic.

Also, at what point would these multiverses differ? At the moment a choice has been made by a human being? But when are choices being made?

While I try to represent the view of incompatibilism, this idea wouldn't work for me.

The universes would not be deterministic, as this one is not.  Each has formed due to the choices of its inhabitants.  They differ not "when" choices are made but "because" choices have been made.

 For the individual observing all available paths, Time is not one-dimensional pathways but a morphous object.  All choices occur at the same time as past, present and future are all occurring simultaneously in the object.  Only our perceptions grant the illusion of a chronological order.

Depending on the choices of the individual, all that changes is one's position on the Finite Curve relative to the others and one's experiences.  As I said, things get crazy when you add dimensions to Time and space.

Think of it like this.  Imagine that this is your life:

You experience it in a linear fashion, progressing from infant to elderly in a steady march.  Now imagine that this is your life AND you are that individual.  You are now looking at the entire continuum of your life.  You see Time from a vertical axis, though experiencing it from a forward axis.  You are a baby.  You are an old man.  You everything in between--always.  This is still only two axes; the third would be a sidelong axis.  In addition to seeing this solitary continuum, you can look around and see all possible variations, from actions beyond your control to every possible choice that you make as well as the consequences.  This would be Time as a 3D object, and it is morphous because you have the power of choice--the shape it has taken is due to your actions and decisions that have affected and will affect your experience in the continuum.

I can't remember exactly, but I think Carl Sagan talked about it in Cosmos.  I know he did it with spatial dimensions, I'm uncertain if he touched down on temporal ones as well later on.

If people still had the same attitude towards freedom today as back in the 1920 and 1960 where they marched down the Washington street to demand total freedom of speech and so on ( Freedom of speech in US first came during these times) then we would have disposed corruption a long time ago.