Oh my god, I think I figured it out...
There is a ritual to swap minds, a vampire into a human body, AND VICE VERSA.
Adelheit's sealed body HAS STELLA INSIDE.
Either that, or Stella was an elaborate ploy to quietly facilitate the relationship between Akabara and Adelheit, and Stella was either an actress, a scapegoat, or Adelheit in disguise.
I need more information on vampire magic to have more theories, though.