CRIME, second autopsy on Adeyooye inconclusive [B]

Pantagraph (Greg Cima):

A second autopsy did not determine what caused the death of [Olamide Adeyooye].

Question for the legal minds out there: how much more difficult is it to get a murder conviction if the cause of death is not determined?

If the prosecutor can prove that there was a kidnapping, is the kidnapper culpable for the homicide whether he killed her or not? Even if he claims, "she was fine when I left her," isn't he responsible for the homicide if he left her in a situation where she wasn't really safe?