The best place to get USC info, outside of people within the program, is from their Rivals premium board. There isn't anyone better. According to them and people I know in CA, Mixon is not going to USC and has all but stated that. The reason being, he's keenly aware of everyone's depth chart. And USC signed 2 5 star RB's last year, and he has told people that while he's not afraid to compete, he realizes that there is only one ball and those guys are really talented as well, so he's looking for a place with a more favorable depth chart (another reason why he won't give ND a serious look). This has been known for months, and the USC fans I know have all but forgotten/given up on Mixon.
So at this point, Mixon is a longshot for USC, and anyone telling you otherwise is wrong, per the people that are the best at following USC recruiting. For that reason I'm not worried about him whatsoever, despite his incredible talent, b/c he won't end up at USC.