Mixon is guilty, no doubt about that. It sounds like the victim has some blame too. I can understand pushing the girl away but not punching her. Too many unanswered questions that we'll never have the answers to.
I agree 100 percent that he had other options and a better man may have chosen them vs striking a female. However, as I have raised my own girls to understand, not everyone was brought up with that mentality.
If a girl strikes a man, she shouldn't be surprised in today's age that she might be struck back. I'm in no way endorsing it and acknowledge there are unknowns, but from the video description, she seems guilty of assault having pushed and struck him first. However, I disagree with you that he is "guilty" without doubt of anything other than retaliation against a female assault.
Could he have left, yes. Would the drunk girl have struck him again or escalated things even if he tried to leave, maybe. Does a man have a right to self-defense, yes.
He is obviously more powerful than her, but had a scrawny drunk guy pushed/slapped him, would we have the same conversation? Likely not.
He lost a year of football over it. Unless, something more comes to light, I agree he seems to have earned a second chance.
If we remember the words of Lil' John:
Don't start no shit, it won't be no shit