The description of the wounds is interesting. Stab wounds in the legs and feet, suggesting she was prone and kicking at him, perhaps from a sitting or recumbent position in the back seat while he stabs from the drivers seat.
Theory: SJ gets into the car and NR either realizes his trap has succeeded or is surprised and decides to use the opportunity he’s been afforded to commit a crime (robbery, rape). If the driver is alone (at this point I favor this scenario, since I don’t believe she gets in the car if there are two) he begins driving away like he’s just doing a standard ride share. Within a few minutes, SJ receives a text from the real Uber driver, at which point she realizes the mortal danger she is now in. She starts trying to open the doors, but is unable to due to the child locks. Maybe she starts to panic and she starts kicking at the doors and windows. Nate now realizes this isn’t going how he has planned. His car is being damaged and she’s screaming at him. He grabs his knife and initially threatens her, but she keeps kicking and screaming. He cuts her, probably intentionally but possibly accidentally. Now she’s really in sheer terror, and he’s full on panicked. It accelerates quickly with her being cut all over during the struggle, including her feet and legs. One or more of the wounds are fatal and the blood loss is extensive, leading to her death in the back seat. He finds a relatively secluded place and temporarily moves her body into the trunk. Panicked, he goes to a familiar place, an hour away, with the intent of placing her body someplace where no one will find it. He purchases the bleach and other cleaning agents thinking he can manage to get out of this somehow by removing all physical trace of her, but for whatever reason he doesn’t have to time to do a thorough job of it. He returns to 5 Points the next night to pick up his gf and is pulled over. JMHO
