I wouldnt play Mangum this year unless I had no other option. IMO we ruined Heaps by throwing him in too early; he was not yet confident. There's a reason Heaps broke all BYU's Freshman records and it wasnt cuz he was that good. It's because before him BYU tried not to play freshmen. It's because Lavell understood how critically important it is to make sure a QB is seasoned and confident in his skills and knowledge of the offense before he takes the reigns. He needs to have great QB reflexes and that takes A LOT of reps, years of reps.
My ideal for every QB is Fresh=redshirt, Fresh=backup, Soph-Senior=starter. That gives two years to learn the system and three to execute. Oh and another key: quit changing the offense!