If I'm thinking right with the pings, it sounds like he left the garage, went home, then back near the shops, without all that much time between stops. Someone please correct me if I'm wrong on that timing! Then, as far as the public is aware, the pings stopped? But LE may have more info that we don't know about yet...
So I'm thinking, maybe he attacked her at the garage, and whether she was killed outright or not, put her in the trunk, drove home either in a panic, or to get something (blanket, tarp, etc.) or just to give himself time to think. Then he drove back to near the shops...IMO to dump something. But if he left Christina's body in a dumpster or somewhere near the shops at that time, what was he doing for the remaining hours until he was seen on video cleaning his car...not to mention being late to work, when he's probably trying to act like nothing is wrong? That makes me think that he may have dumped her things there...phone, etc....getting "smart" at that time, and destroying her phone, maybe disabling his own, too, so that they couldn't be traced. If so, her things are probably in a landfill, never to be seen again. But it's always possible that he just went back by there to see if anyone had noticed anything wrong/cops around, etc. This is all completely JMO, trying to brainstorm what he might have done!
It has also occurred to me (as it has to many people, I know) that if a body had been in his trunk for any length of time, decomp would have been detected. Could he have attacked her initially at the garage, panicked and put her in the trunk, then in the most colossally heinous and stupid move ever, killed her later (possibly even in his own garage, at home) just so that she couldn't "wake up" and tell anyone what he had done? He could even have switched to a different car at that time (as has been previously suggested, by other posters), which would explain there being evidence of Christina having been in his trunk, injured, but no detection of decomp there (that we know of). He could even have placed her on something before killing her, reducing the possibility of evidence being left behind. That scenario would be so cold-blooded and evil, it makes me feel nauseous just thinking about it.
After the bond-reduction hearing, I personally feel even more strongly than ever that EA is responsible for her death, and that finding her body hinges on his actions after leaving his house. I really don't know whether I think he had help disposing of anything. But I do think that whatever he did, he did it before going to work that morning. I don't think he knew how long it would be before she was reported missing, and I think anything he did after that time was just clean-up.
It isn't my intent to argue against anyone else's theory with this post -- I'm really just trying to organize my own thoughts and trying to think what he could have done with her, and typing it out helps me get my thoughts straight! And I'm really not sure on the motive for attacking her...purely for a SA...drugs...long-hidden obsession or hatred...I really don't know. But Christina is out there somewhere, and I do think EA is the key to finding her. I only hope that he left some kind of evidence behind that can lead to her (be it more phone pings, texts, physical evidence, whatever) and that finding her won't depend on his "good will" in telling where she is. All MOO!!!