Does free will exist?

Would knowing change anything?

Only if determinism isn't real.

Now to seriously hurt your head.
Is the actions of 1 pre-defined, based on the prior actions of others, or not?

Will is in custody

No, if you know everything in the universe.

Yes, for now.
The view of a deterministic Universe is no longer true thanks to our advances in quantum mechanics, but maybe future development of the theory would yield hidden variables or methods that dissolve current uncertainties.

Yeah sure, as long as we don’t know how photon moves, can’t solve three body problem and think that universe was born from big bang.

Free will is just tied to time, get rid of time you get rid of free will.

Unfortunately our whole civilization is built on time concept and nobody is brave or smart enough to provide reasonable alternatives.