A failed try is over once the kick is blocked, the ball is dead or the defense gains possession. The defense cannot score in any way. A blocked PAT is dead and cannot be picked up and run by either team, unlike a blocked field goal which can be picked up and advanced by the offense if the ball is still behind the line of scrimmage, and picked up and advanced by the defense anywhere.
The kick in the Auburn-Alabama game could not be returned from the endzone like that one, but if the ball came down anywhere outside the goal line, it could be. Once a scrimmage kick breaks the plane of the goal line it is dead. If it goes thru the uprights when kicked from the ground or a tee, it earns 3 points. If it's a punt, no points (obviously) but other than that, there is no difference in the rules between an attempted field goal and a punt.