As far as Nelson goes, I really liked him. But if he can get a redshirt for this season, Bronco should try to bring him back and convert him to another position. The kid is athletic, smart, and plays with a ton of heart. He could be a real asset to BYU in other places on the field.
I'm thinking there's a reasonable chance that Heaps will go on a mission. Yes, he says he's not planning to, but after a year at BYU, going to a student ward where most of the other freshmen guys will be getting their calls, hanging out with RMs on the team, etc., I wouldn't be surprised if he changes his mind. In that case, I'm not sure what other options for quarterback they would have besides Nelson.