Can you get Celeste a different cage that is lower to the floor so you can use ladders where she can get back into the cage on her own?
I have a huge flight cage (Prevue Hendryx 050) and my female isn't able to fly either. I have ladders set up from the floor to the bottom open bottom door so she has the ability to get back in the cage when she ends up on the floor.
I wouldn't worry too much about her trying to scratch her head inside her bell. Many budgies put their heads in their bell toys - seems to be a fun thing to them. Unless she starts regurtating to the bell on a regular basis, things should be fine with it.
I'd continue to keep her away from the other budgies so they cannot seriously injure her if/when they become aggressive.
Spend as much time with Celeste as you can. Simply interact with her when she is inside her cage. Talking, singing and reading (your homework would be fine...) will help her feel as if she is included and she won't be as lonely.