I have thought this scenerio was possible but then why the sudden stop of communication from Christina? It really does not make sense. She was preoccupied the whole evening trying to get in touch with HF. Calling and texting him with no response. So thats what gets me.
You make good points, but I think some of it is easily answered, some less so. Then, we have to consider what possibilities that leaves us.
I think part of the equation was that her phone ran out of juice. Or was about to run out, or she wanted to save the last bit for her drive home, something like that. Didn't she have one of those iPhones that drains a battery like mad, and hadn't she been in it constantly for an hour or so? In addition, that offers a reasonable explanation of why she borrowed EAs phone as she walked to the car.
So she gives him his phone back, they get to the car(s), and now what?
Well, imo she couldn't talk/text unless she uses his.phone. Or gets hers charged. So those are my assumptions and that leaves us with the following, I think, in the order of what I see as most likely to least (my thoughts about how other evidence might fit together influencing my thoughts here, of course) ....
1 passenger - she doesn't talk/text because borrowing his phone to interact elsewhere while she was riding in his car would be rude, she doesnt even ask
2 passenger - she doesn't talk/text because she asked but he won't let her borrow it, and maybe he's using it
3 passenger - she doesn't talk/text because they are lost in conversation
4 in trunk - she doesn't talk/text because she can't
5 passenger - she doesn't talk/text because she is disinterested in talk/text for awhile