Hitting someone and driving away is a crime. Marshawn did hit someone, and he did drive away. Or at minimum, someone driving his car hit someone and drove away. Just because someone has the right to not incriminate themselves does not mean that they did not commit a crime. If you're going to dispute that this is what happened, I don't want to argue with you, it's not worth the effort.
There was speculation on this board about what was painted on his lawn; what's come out since then is that it was not a racial epithet, but basically said: "Learn to take a knee" with the score of the game, and a graphic depiction of the male anatomy.