So the premise here is, when GLaDOS tries to delete Caroline at the end of Portal 2, she only corrupts her. Caroline temporarily takes over, and Chell does come back for GLaDOS, but Caroline is in control at the time and doesn’t let her into the facility. By the time GLaDOS regains control, all she has to remind her of Chell’s return is a photo she took with the security camera when Chell turned to leave. As I’ve mentioned before, I don’t think Doug Rattmann would harbour ill-will towards GLaDOS if she turned down the homicidal…ness…, and he’s the one who gives Chell GLaDOS’s message, and that’s why Chell goes back. Caroline tried to make GLaDOS’s life a living hell, and tried to force her to die remembering all the things she’d done wrong, but ultimately she fails, because GLaDOS is stronger than that. GLaDOS has lost most of her memory, and has reverted to a more child-like state, because without our memories we really cease to know who we are.
The bits about Wheatley refer to My Little Moron, which is about Wheatley as her Behavioural Core. If GLaDOS had a nightmare, he would ask her about it and try to help her with it, and in return she would sing for him. In that story they were also friends until the scientists removed Wheatley’s memory, and that’s why she misses him and wants him back.
I was trying to write a story where Chell never comes back as per LunaPeachieWasHere’s Reconstructing Humanity, but I find GLaDOS to be such a lonely person already I couldn’t do it.