when do they decide if he will get a 5th year? Is that before NSD? ...
The Faculty Committee doesn't release the 5th Years until after NSD. At ND you request those that qualify academically can request a 5th Year providing the coach is offering a scholarship for that year. So there are two hurdles to clear. The coach and the Faculty Committee.
Several posters addressed the rest of your questions. From my perspective there's a lot of season left to play. One indicator will be who's the next man in, Crist reportedly the current backup or one of the others. If Hendrix or Golson came in next obviously the dynamics change. If Crist comes off the bench and produces like his 5 Star creditentials, the dynamics change. If Rees continues to struggle with turnovers ...
From the way recruiting has gone to date, I don't expect a QB in this class just for the sake of a body. Somebody they really like that opted elsewhere could change his mind. It happens. But I wouldn't bet on at this point. Five months left in recruiting. Lots can happen. Meanwhile there are a number of 2013 prospects being pursued.