Ms. Boom is the WorldSmith
To start this theory, we need to know Deltarune's theme. Escapism. This is reflected in how the Dark Worlds work and in the characterization of people like Noelle. I believe this theme makes Gerson Smith impossible, because most theories require him to either be unconscious or not fully himself. But who would use the Dark Worlds as escapism, has a Testudines shape, and has a ton of evidence going for them? Ms. Boom! Before you comment, hear me out.
* The three Dark Worlds were either near the Hospital or where Ms. Boom was last seen. The School is two houses away, the Library is two houses away, and she was at the Church. Even while sick, I believe she could manage to walk two houses.
* If Susie doesn't introduce Noelle to Ms. Boom, she will act more distant toward her.
'She's in a hospital bed' She managed to get to the church, and the WorldSmith floats, which alleviates that.
* The Smith's breathing sounds heavy and raspy. Like, as if she were sick.
* Toriel's STEELSHARD dialogue mentions an 'Old friend' and she shows a deep mentorship-like relationship with Ms. Boom.
* The Smith's presence is tied to illness. In the fight, most of the flavor text mentions feeling sick, '* You felt your chest twisting' and holding your breath speeds you up.
* Ms. Boom and the Smith share a meticulous personality.
* What if Gerson were the Smith? Who is the voice in the code?