In the spirit of an open mind... here is another hypothetical scenario that fits the few facts and tidbits we have. IMO!
What if KR knew she couldn't drive home in the condition she was in. People say she was a very responsible kid. Maybe afterall she decided to stay the night at the campground in her car. So KR lay down on the back seat of her car and passed out, after turning off her phone at 12:33am so she is not disturbed.
At some point later in the night along come two guys (or gals). They are very intoxicated and looking for a car to joyride in for entertainment. They look among the parked cars remaining at the party to see if someone left a car with keys in it.
Voila, they find one. Its dark so they don't see KR sleeping / passed out / unconscious in the back seat, maybe under a blanket. Off they go, driving around the dirt roads near the party and down onto the beach. They get too reckless and end up in the water. They manage to get out but KR does not. She may not even wake up. She could have been injured (neck?, head?) during the wild ride. And sadly she perishes. In the dark, the kids don't even notice. All they think they've done is dump a car into the Prosser Lake after some fun. Until they find out KR is missing... and her car.
That long, pregnant pause by the Nevada Captain at the 8/22 presser, when asked if there was more than one body in the car, could be because he knew there was a second person in the car when it went into the lake. Maybe he knew that because both front air bags were deployed. Only LE ( and AWP) would of course know that.
This is hypothetical, conjecture, speculation and all that. But what if the incident that killed KR was an accident, but just with a very complex set of facts.