I guess my opinion is that CM left with EA in the passenger seat. Maybe they just can't prove it, due to the fact that EA did a sufficient job of cleaning the passenger area of the car. I also think her phone was probably dying, or had died from all the call attempts on that walk to the garage and usage from the night. I think that is when/why EA texted HF about looking for "that good rock" My guess is that as they got to the car, they had no response from HF, that EA and CM got into EA's car to go score somewhere else. Maybe CM said she knew somebody or felt obliged for some reason to help EA get that good rock...but for whatever reason, I feel like things were still okay at this time.
I think that it was on the way back that things got ugly. Maybe it was a rejected advance, maybe it was frustration and anger over driving around trying to score and failing, maybe it was a deal gone bad and he blamed her...either way I think as he was about to get CM back to her car things went wrong and quickly. Maybe he turned around refusing to take her to her car and she grabbed the wheel, causing him to swerve and damage his vehicle. In a rage he he hits her and grabs her with his right hand while regaining control of his vehicle. It is during this time that he sustains some of the injuries such as the bite mark. Maybe she got free and he felt had to stop her and that's when things got bloody and she was placed in the trunk. There are many scenarios to speculate about here, but I feel as if this was the most likely time for things to have taken a deadly turn. I also think it was at that time that he possibly turned her phone on to see what messages were there, and it pinged for the last time. He then most likely destroyed or disabled the phone permanently. It was at this time that he began actively covering up his crime. All my own opinion and imagination.