I'm not even sure who the 5% unknown is. I know who raises my hinky meter, not as much as PF but enough to go "hrmm...."
But ok.
11/22 - PF and KB swap the baby. Maybe they squabble, maybe not, whatever.
11/23-24 - unknown. May just have been too long to ping her cell, people go on radio silence for a day or two all the time if they want, maybe she has a second phone we haven't had reported yet, too many other things.
11/25 - Unknown tells Kelsey something. Who knows what. It's enough to get her to grab at minimum her purse and phone and leave. At some point, she texts PF and her employer. Around 5 pm, they are near the Gooding Idaho tower that pings.
She's killed at some point after that. Why, don't know. Who, don't know.
It's an unknown to us, it's a known person to her. Work excuse, family excuse, whatever reason that had her willingly get in a vehicle with another person.
95% likely it's PF. But 5% could be someone else, who knows full well that everyone will look at PF hardcore because that's the statistical normal. And they got dang lucky by having an extended gap before anything was reported to the police.