There's absolutely a scenario where Scalley get the head coach job, and brings Jay Hill in as the DC.
Maybe it's money, making it's loyalty to his alma mater (and 10+ years of coaching history), maybe it's personalities, but let's say Hill takes that job.
Then it's also quite possible that BYU moves on from Kalani in a few years and brings Hill back as the head coach.
But the reason that's really believable (moving on from Sitake) is because BYU would likely struggle after many of the best defensive players and recruits went with Hill. So even if Hill ended up in the role, there would be some real pain in the process of getting there