The two are not inconsistent. I believe all four received specific life-ending injuries, and I believe at least three of them will have very similar fatal injuries (perhaps all of them). It's possible that one of them (Kaylee) received more injuries in addition to the ones mentioned by her father.
There was no recovering from the organ damage done by the murderer, IMO. And they were dead fairly quickly. But it was not instantaneous, unless the existing information from the Latah County Coroner is in error. Fast, but not instant.
IMO. It's interesting to me that from what we know, the injuries were all within the strike zone permitted in boxing.
Note that some awareness of where organs are placed is needed.
The liver is not off limits and every good boxer knows to protect that area of the body, and boxing injuries to the liver are no joke. Apparently, the world of MMA knows about liver punches as well.
As we know quite well, combat sports and martial arts have some very specific and special shots, be they punches or kicks. Each one has some peculiarity, but
wayofmartialarts.com
en.wikipedia.org
But the neck/back of the head area is off limits.