Bohanon is one and done. Retzlaff has two years. Bourget has three years. Hillstead has three years plus a red-shirt year.
Bohanon doesn't really have upside (as opined below) because it is now or never. He is also a stop-gap. Nevertheless, if he is clearly the best, you play him.
I don't really care, but I hear that Retzlaff is the team leader. I would bet they start him for this reason alone. Leadership in year two is key. Also, if he is good enough, you could have him play most of the season giving way to Hillstead for a four-game red-shirt stint, Bourget, or finishing it out.
Unless Bohanon is clearly the best, he is your super experience back up in case Retzlaff goes down or doesn't perform before Bourget or Hillstead are ready.
I don't think this staff will upset program development for a one-and-done QB unless he is elite, or there is no other choice (Retzlaff is injured or drastically underperforms).
Remember, Bohanon sought them. The didn't seek Bohanon. Now, I could be wrong and Bohanon could end up being just the X Factor the team needs. But that is just not how I am reading the tea leaves right now.