It seems like we split on the final hit on the Diva, however, the game does not save(have a record of beating the game) until you hit the Diva's lips that fall off of her. Should the final split be on the Diva hit, or the lips?

The final split should be as soon as you kill the lips of Diva. It is that way because of that's the last point there is player input and that is when diva reaches zero health.