(1) Does the TD signal blow the play dead? I can't find a ruling either way on this point, so some kind of documentation would be helpful here. Additionally, did the refs signal the play dead when the TD was signaled? This would make the previous question moot. Regardless, if the answer to either question is yes, the play is dead. Even if you call the TD back, you can't penalize Mathieu for what happened after the play is over, resulting in a first-and-goal inside the one.
(2) If the TD signal does not blow the play dead, this is bad for college football. If a TD is called but the players involved believe it may be called back, this will encourage players to continue to play after the TD is called, increasing the risk of injury as some players believe the play is over and others do not.
OK, flame away.