By now aqua could have had like a bajillion ways to escape the realm of darkness. Everyone else that we know of escaped from it - Riku, Mickey, Ansem the wise...but why not aqua
A conversation between Vexen, Zexion, and Lexaeus enlightens us on how there are ways to escape -
Zexion:"His existence - it was once doubled in the darkness."
Vexen:"Fascinating... That's why you mistook him for the Superior. The dark power given to Riku facilitated his escape from its realm."
- from this information, using the darkness can help you escape from the dark realm. Vexen continues as to why he appeared in C.O.
Zexion:"What I want to know is why he appeared here in Castle Oblivion."
Vexen:"That's really quite simple. His existence resonates with that of another hero."
In an earlier cutscene, we see Riku using a card to escape.
Mickey uses the same thing to reach Riku.
So why can't Aqua get out? She could use a card as well, and since Vexen's theory seems to be correct, she would be able to appear in C.O because Ventus is there, AND his heart is inside sora's...so why doesn't she get out?
Also, why doesn't she use darkness? Ofc she is like those total darkness-phobe, but every heart consists of light and darkness. Crap, sora even uses the darkness frequently.