Whoever the 2's are? Surely they were taking reps for the prior month of Bowl Practice.
I'm with you, it's not an ideal situation to have backups in. But at least a series each half should be doable. Whether you put all the 2's in at one time so that they have continuity from practice or just one at a time, it's going to be helpful. Asking starters to play 100 snaps and be able to hold up late, under pressure, is unrealistic.
We all knew coming in that MLB was a concern. But these are 4/5 star guys, they should be able to play a series of football and not look silly. (Holcombe/Shy Carter/Deionte Thompson/Mack Wilson). And you're right that there wasn't enough game experience for those cats... Especially when you're talking about almost an entire season of domination. How can we not have a backup at each position to sprinkle in a few snaps? Injuries are bound to happen... Should be taken into account all season.
Ultimately they're all going to make mistakes. The starters were awesome when fresh and beat down in the 4th. By nature the backups will likely give up more yards, plays, scoring opps, etc. But the idea is to give your studs a fair chance to finish the game. If there's any team in the country that should not have to play their studs 100 snaps its Alabama. I'm thinking the staff will take a serious look at this and adjust moving forward. All those assistants, analysists, and hours spent... This has to come up.