So, about Crescent's appearance in Nightmare City.
She says "Not much time left" over and over, so I believe she is being chased by the two masked people in the real world. (do they have names? can't remember) She's in Nightmare City because she's afraid and that's what nightmares cause. Fear. She wasn't immediately killed, but was being chased. Since the two masked people have strange powers, they might be able to disable her teleportation or something. So while she was alive then, she was killed by the time you get to the pyramid. Though.. she IS seen there still if you revisit after the end of the current main story. But.. you could stretch it a little bit and say that Zetta was turned into a Solosis and is there so dead people could appear there since Zetta isn't in his human form. But hey, that's just a theory. A stupid theory.