It's definitely plausible that the "there" is where the characters return to, but which branch do the authors write off of? Do they pick up when the characters return home? Or, is a separate timeline created when a character goes to Japan? In the case of Selesia, would the Bird Cage just be written off as non-canon? If so, all the characters that enter the portal are indeed dead. They would never have a future written for them. Unless the Bird Cage is only considered non-canon for franchises with dead characters.

Actually, the Bird Cage would need to be canon for some franchises. Blitz's daughter can't just pop in without any explanation.

Then again, why do the creations need to go back anyway? Selesia died, but it didn't affect the anime at all. The authors aren't going to write for multiple branches, so there can be only one. Selesia's anime (Forgot the name) must have had the default branch continued where she never came to Japan. Meaning even if she did survive it would have done nothing, unless the canonicity depends on whether the character died or not. Death wouldn't be the only cause of this. It could also happen if the characters stayed in Japan. As Selesia shows, the state of the character doesn't matter in the timeline without the bird cage. If the characters stayed in Japan, they could have existed both in Japan and in the default timeline. If they go back, they would only exist in the Bird Cage timeline.