Could Happen, but Getting Ugly
This is where things start to get a bit muddy. If Dart decides to come to BYU and not serve an LDS mission, you can say goodbye to Jacob Conover right now. I see no realistic way that Conover stays on the team being a third string quarterback while entering his academic junior season.
In this scenario, Dart likely becomes Hall’s backup with the understanding that Hall gets injured a lot and once he does, it may be his job to keep.
Why this won’t happen: I don’t think Dart is going anywhere that is telling him “you may be the starter”. Honestly, he’s got the opportunity to go play for a Top 10 air-raid offense at Ole Miss. He isn’t going to turn that down to be a backup at BYU or anywhere not named Alabama.
Why this will happen: The only possible way that I see this panning out and working is if Dart takes the perspective of understanding that Hall likely will end up missing games next year and the risk is worth taking to be at BYU because he feels it is the best place for him (being close to home, QB development).