a. if true (biol one sex with brain characters of other sex, obviously brain diffs btwn sexes is true) and is a prevalent feature of transgender persons, that very much argues that the condition of transgender is a disease.
b. it is not necessary that there be any racial difference in brain characters in order for a person to believe themselves to be of a race that they are not. it is only necessary that 1) race be a component of a person's identity, and 2) that it is possible for there to be discordance between what others consider to be your race and what you believe it to be. whether you consider the font of that discord to be subject to voluntary control or not is not particularly relevant to the consideration of whether it is possible for someone to believe themselves to be a race that is not the same as the race society believes them to be (i'm being a little careful with language, since race, as constructed in the us, does not align completely with groups of distinct genetic lineages; however, in most cases the difference is probably not material).