Regarding Christina's backpack and phone. If Christina managed to fight her way out of the car, she might have dropped it to keep from slowing her down.
Now trying to think like a immature 23 yo (some folks still say I haven't grown up): If the person has gone far enough into a secluded area, I leave them where they lie and minimize contact. Otherwise, drag them nearby where it won't be so obvious. I may be dumb and immature, but I don't want trace evidence in my new car. Bag, yeah, maybe leave it where it fell. But if it fell in the dark, I'm not going to bother looking for it. I need to clear the area fast before LE shows up. Bag is in the car, well then I take it to a dumpster off the highway or some other site somewhere. And just in case it is found, I'm not going inside it. I don't want my DNA in the bag. The phone. I make sure it is off and drive to a bridge and toss it off in the water. I DON'T want it being found. Otherwise, if the person dropped it in the dark, I probably never find it.
All of this takes place in panic mode. Assumes no premeditation and therefore no careful planning. We're not talking about a trained assassin here. An ordinary person who wants as much distance between them and the victim/crime scene/personal effects as soon as possible.