I'm now thinking he'll have to go full Groundhog Day - that it's not about speed (since she can have been a prepared vessel even before he set out to begin with), but about quality (that if he can juggle all the cats together and 100% the story then their combined abilities might be able to prevent the demon lord from actually possessing her after his death).
But we'll see (I hope), the story is well-wrought so far so something entirely different could also happen.