Proving free will exists is like proving red exists. You can point to the wavelength (or the brain signals) and declare “that is red” (or “that is the brain signature of a person deciding”) but neither of these conveys in any way the actual subjective experience of perceiving red or making decisions. Note that even if you were to, say, take a colorblind person and force them to see red by directly stimulating the relevant portion of their brain, that STILL does not neatly encompass everything there is to know about “seeing red.”

Also, what does a total lack of free will mean for the criminal justice system, which operates on the unspoken assumption that behavior is at least in part willful?