I think that if a fight occurred, especially involving others as well, in the garage,there would have been some signs. Also maybe tiny paint chips if a car was damaged, etc...
I can't quite jump to premeditation if it was all EA, as I am baffled by his seemingly casual body manner, walking style, etc. especially if he was drunk. I tend to believe he probably was, if they were out for so many hours.
Anyway I am not into a group scenario as yet either, except maybe a "helper" later. I still think this was a one-on-one crime. Jmo
I believe a fight occurred. But btw EA and Christina. In his car. Based on the cell activity things happened fast in the garage and because LE kept the passenger seat belt & floor mats Christina was in the front seat. I believe EA either (1) offered to drive her to parents in Allen since HF was not responding or (2) the ride in EA's car was supposed to be an out and back drive to show off his car. Hence Christina would not be alarmed at the direction they took away from Ft. Worth.
As to EA's injuries, that would seem to indicate something happened btw them in the car. How the car was damaged and where it fits in? That could have been before, during or after the incident btw them - I can't tell as it may be something not directly related to her disappearance. I still believe EA made a move on or an assault on Christina. Or something she found highly inappropriate or threatening.
I don't believe anyone else was involved unless someone helped after the fact to cover it up. I don't believe it was premeditated.
But just my theory.