Can I put a rejected joey back to the same cage as her mother?
When she's fully grown of course.

you'd have to go through the introduction process like you would with any two gliders and hope that they got along... but as long as they did, there's no reason why you couldn't put her back with mom (assuming dad had been neutered, ofc, since you don't want any inbreeding).

Yep, what above said... dad would have to be neutered thou... otherwise he will breed her, and then you would have a mess on your hands

I also want to say...this girl should NEVER be bred (but esp inbred) as she had such a hard start on life, IMHO you would not only be risking her life because of joeys taking to much out of her, but also risking joey's lives