IMO, the only reason that either would transfer this year would be ego. Saban is not going to name a starter until at least the fall - and maybe not until the season begins. That in mind, they are both far better off staying at Alabama in 2018, whether they be named starter or backup. Neither is likely to start at another school in 2018 if they transfer. 2019 is the year in which you lose one or the other, IMO.
Maybe. If I was Tua though I would be trying to read the tea leaves to make sure I had a better than even shot of winning the job (I think he does). Even if you don't start somewhere else in 2018 you at least get a year to learn the offense before starting so you aren't just shoved into a new offense and asked to start right away (see what happened to Coker for example). Again, this is just how I would look at it if I was him. I wouldn't want to waste another year sitting behind a player that I think I am better than and isn't going to leave early for the draft.