Pure speculation on my part, but I feel like the starter will be Buchner. I don't think you go to the transfer portal and bring in a guy who was the starter at Notre Dame last season prior to an injury - and who also played for our current OC - unless you fully intend to play him. His career stats are underwhelming and he seems to be somewhat injury-prone, so I'll admit it looks like a head-scratcher at face value.
That said, my guess is that Saban & Rees didn't see enough of what they wanted from Simpson and Milroe in the spring, and Buchner was available. I would guess Rees convinced Saban to go get him.
I'm guessing they both know something we don't. I have a hard time buying the "Buchner was brought in to push Milroe & Simpson" narrative.....I just don't think the optics match up to that.
Again, this is total speculation. I'll add that I think the offense may end up being more "QB-friendly" with Rees. I kind of envision something like the 2014 offense under Kiffin. If what we run this season resembles that, I think any of these QB candidates could be consistently successful.