I agree. That is the most confusing to me as well: the traveling north on 121 and then returning. This is really the part that makes me question if others were involved and/or she went willingly. To me it makes more sense to think she agreed to ride somewhere with him and then he made some excuse to stop or pull over and something happened then. Or they were going somewhere and either he got a message from someone (or he pretended to) and that's why he double backed. Of course how do you explain the thoughts of a person who appears to be responsible for someone's disappearance/murder??? It's hard to put logic to work here, if you ask me. But I just try to piece things together somehow.
If her phone actually did die, and she had been aware that EA texted HF for some "good rock," for all we know EA could have told her that HF responded and said "Meet me at that parking lot north of 121 and Alma." And even if it weren't true, maybe CM fell for it and EA said, since he's pissed and won't answer you just ride with me over there, he'll have to talk to you once he sees you there and you can make up and stop fighting. Maybe that's how he got her in the car. Then they exit 121 and sit there waiting and then he makes some move on her and that's when it all happened. Then he thought he'd bring her to the Shops to dump her off but those cars were leaving the garage so she decided to drive back to Allen.
Just spit ballin' ideas here. Trying to make sense of the timing of texts and lack of cell use and phone ping timing. All just my "creative thinking."