Romney has demonstrated he is a cool customer with a fast chip. If the O-Line is average or worse at pass-pro, he's your guy. If not, he's your #2 because of his mastery of the offense and proven ability to come in cold and perform.
If your OL is better than average at pass-pro, then you take a chance on Hall. Let him go until he gets hurt or loses his mojo. And if neither happens, that's fantastic.
It's very exciting to have someone as highly recruited as Conover. I think he's gunna be great eventually. But where there are 2 proven guys ahead of him, I wouldn't rush him. I particularly wouldn't rush him in mid-game if Hall has gotten hurt. There is no reason to do that when you've got another (either Hall or Romney) who has already played in D1 football games and who has many more practice reps in the current BYU offense. The only way I'd bring Conover in this year is if BYU is out of contention for a bowl and in complete rebuild mode.