Yeah, on re-reading it's clear the shelf is shown as if it's for a taller person. At least a few inches taller. Same with the tool placement. Her biomom? An older relative? A longtime staff member? An assistant torturer? If it doesn't predate Jelly it's built for more people.
Maybe Jelly really
was possessed, after a ritual and blood writing. Which could have killed the original soul. Who might have been reincarnated by now. As a slime. Like the one who has extra buffs after taking over the body.

There's something weird going on...