Also where in the brain does this free will exist and how does it work in practice, there's no mechanism that somehow influences the trajectories of your brain signals on the fly, just chain reactions

Bah, prove to me that you're sentient. Illusions, all of it! Well, it can't come from nowhere, perhaps in every particle there's a quanta of consciousness with potential to arrange into patterns that enable higher cognition.

sentience pops up along the way even though it's never there to begin with

Yes, many events become less probable on higher scales, not vice versa. There is no reason to assume choice should pop up along the way when it's never there to begin with though.

“Your Honor my client pleads ‘Free Will is an illusion.’”

On a macro level, time reversal ain't so easy.

I don't know what experiment you're referring to, but what occurs on a microscopic level does not automatically translate to a macroscopic phenomenon. For example, any particle collision can be time reversed.