This isn't hard. Frank Maile. John Rudzinski. Lance Anderson. Morgan Scalley.
I could go on. There are more than a few LDS guys who could be interested, and even more guys with connections to the Mountain West region. It's hard to believe that a defensive Jeff Grimes isn't out there.
Additionally, the "Who else/there's no one else" argument is defeatist and intellectually weak. BYU isn't a bottom-of-the-barrel team, and there's no reason that coaching searches and coaching hires should be approached with that mindset.
I'm also puzzled by why you're bringing Tom into this. Do you feel like Tom is close enough to Tuiaki that the "fire your own mother" thing is relevant to Tom? Regardless, I suspect that the decision to fire Tuiaki is Kalani's more than it is Tom's. It's very possible that Tom is fine with keeping Tuiaki because he feels like keeping Kalani (and keeping Kalani happy) is, on the balance, better than firing Tuiaki. Be that as it may, it has no bearing on whether Kalani should "fire his own mother" by letting Tuiaki go.