Which unidentified male?
I think the most likely scenario is that Nick did something to her that night. If he really was back around others in a short period of time, then it's likely he had her in his tent or his trunk.
He likely said she asked to be dropped off in order to explain why neighbors didn't see his car in the timeframe he told cops he took her home.
I think the phone pings must indicate that he didn't drive close to that other tower near her place before disposing of the phone. Chances are, he also disposed of her close by, although he might have been smart enough to ditch the phone separately. He still showed up in public, so that's pretty likely, unfortunately.
Who knows how long he kept Kelli
His car may be crucial in this scenario. Her body has to be somewhere, and unless really close terrain suggests a good hiding spot, he drove the body. Septic tanks, garbage cans and the like are crucial to check, too, of course.
Motive is another violent sexual attack, except now he knows better about prison
The last girl he did this with was apparently an impulsive, stupid crime (as well as a hideous one). There's no reason to believe, imho, that he'd be more in control or careful this time. The fact that she is missing shows he worked at being careful -- by escalating to murder! Just mho, but I'll be terribly surprised if he didn't do it.