Well, our "pre-team L3" and "competing L3" work out together, so that particular scenario wouldn't happen at our gym.
. If the gymnast has her skills competition ready by season, she will get to compete. Every now and then the coaches will allow a gymnast to scratch one event in L3 or L4... I've even seen it happen once in L5.
Once in Optionals, there's no scratching. If you don't have all the required skills for that level, you don't compete it, including L7 (our gym does not compete L6).
For the levels above 3, you are placed in the "training Level X" group after season has ended and the idea is that all kids in the group would ideally move up to the next level. It doesn't always happen. If the gymnast cannot get the required skills in time, they get moved back. Some levels work out together so it's not too noticeable.
The coaches try really hard to place the girls in the group they feel will be most likely for them to have success at, in some cases they're taking a chance and seeing how it goes as the girl may be on the cusp. Sometimes they know right away that the kid will have to repeat, sometimes they're giving the girl a shot to make it.
In your case I would definitely ask the coaches what will happen if she doesn't get her skills in time, especially if the team and preterm work out at different times.