Your theory makes sense and assuming it’s true I have a somewhat additional theory in which the demon king’s soul really IS the one in Yooan’s previous body but he locked his memories away so that no one could track him and the soul that the evil dude has is actually Yooan’s original soul that the demon king swapped with him, which is why he is still somewhat connected to the trapped soul, because he stole Yooan’s memories from his soul in order to act like him. I’m also pretty sure the vessels don’t know that they already have the real demon king’s soul which is why they’re so desperate to free the trapped soul.
I have another theory. What if at the last second before the demon kings soul got sealed he split his soul? Then it could explain the fact that the 2 souls are connected and that they are compatible to the body.
i have a theory that the mc is actually the demon king.. and he lost his memories. but the only thing thats throwing this theory off is the fact that another soul is sealed.. i think hes the demon king cause of the way most of the other characters treat him, im talking about the red hair and black haired dude the ones that actually know he is from another dimension. idk its weird cause the way they talk to him sometimes makes it seem like he was actually the king, and theyre like overly sensitive about how he sees them.