Zelda is able to return to the realm of light because Midna returns Zelda's power to her. You see her do it in a cutscene after exorcising Ganondorf from Zelda's body. And I know you'll ask "But if Midna returns Zelda's power to her, how does Midna exist in the light?" It's because Midna has the completed Fused Shadows which protects her. Midna actually explains this to Link. She outright says that because she has this new power, she can return Zelda's power to her. She doesn't need it anymore.
You see, without the Fused Shadows, Midna cannot return Zelda's power to her or she'll die in the process. So when Zelda sacrifices herself, there's no way to bring her back. So Zelda is effectively "dead" as in she can't come back unless Midna wants to undo her sacrifice by killing herself. They can keep sacrificing themselves endlessly by passing around the power like a hot potato, if they want but that's not a solution. When Midna gets the Fused Shadows from Zant, the first thing she say to Link is that they have to save Zelda. She immediately realizes that now she can safely give Zelda's power back to her. Midna does not fully comprehend the power of the Fused Shadows until that moment as she's never used them all before. It's very clear that she doesn't if you watch the cutscene. That's why Midna doesn't bring up saving Zelda immediately because she doesn't think she can and all they can do is try and stop Zant.
Now is there a scene where Midna explains it all out exactly the way I did? No but it's easily inferred by what she says upon obtaining the Fused Shadows, I mean, she straight up says as much. I think people are just not used to having to infer things in video game stories from context. That said, you clearly have forgotten some plot details or dialogue from the game considering you forgot that Zelda was trapped in that room and that Midna returns Zelda's power to her, which are things that are explained clearly with dialogue. Not sure it's really fair from that position to say what I explained above is vague theory crafting when it's literally just what happens in the game. Was it a little sparsely explained and could've been more explicitly, repeatedly and openly stated? Sure. But not explained at all? Not true. Also did you really think they were going to permanently kill off Zelda? I think the idea that they planned to and then decided at the last second to bring her back doesn't really have anything backing it.
To address something you said earlier in the thread, about how you don't understand how people can like the game's story, I feel like there's plenty of reasons to like it? Like even if what you said is a plot hole was a plot hole, it doesn't make the entire story fall apart. Same with the bublin's kidnapping the children. I don't think they did it on any orders, just on a whim because they're monsters/outlaws. It really doesn't make the story impossible to understand. The basic plotline is pretty easy to follow and it's full of great set pieces, character moments and character development, all wrapped up in a pretty cool vibe with classic Zelda charm. It's not really that hard to understand why people would like the story in this game. It's a bit more involved than Zelda tends to be in a good way.
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