So...if Ansem forgot how to use the corridors, or he was purified by KH like Riku, that would keep him effectively trapped in the RoD. I wonder if he could possibly remember some of this over time and allow himself and Aqua to escape again? I mean, you don't hafta be dark to step through. Kairi proved that. So maybe since Ansem could remember...that would clear up Aqua's and his problem.
If he originally learned how to open corridors in the RoD, he could do it again. I'm sure exposure to extreme light would fade over time...possibly.