Must we accept that for all cases of mental phenomena ... the brain scan test constitutes a proof?
I think the important philosophical question here is "A proof of what?" Suppose that science did somehow establish that all of our choices are causally determined by our earlier brain states. According to compatibilism, that result wouldn't threaten free will at all, and according to many compatibilists it would be good news indeed for free will. Before we get too hung up on whether brain scans are evidence in favor of determinism about human choices, let's ask the prior philosophical question "What difference would that make for free will?" You'll find this issue discussed many times on this website, including here: