No one is "inserting fiction" into anything imo. There are misunderstandings and lack of knowledge, though.
For example, I would wager that CM didn't call (or get called) specifically to inform that she was walking to her car. "I'm taking a step." Then another call "And I am taking another." Nope, don't think that was really the actual intent of the call itself, but perhaps a comment within some other content, that was later deemed noteworthy because it revealed some of her path when looking for what happened to her.
Much later, we outsiders hear that it's known she was walking to the car at X-o'clock because she said she was, in a conversation at that time - and it gets regarded as being the main point of the conversation rather than an idle comment within some small convo.
"We really have no way of knowing yet why or how Christina ended up in EA's car."...<< I agree with this. But we do know she was in it eventually. The car left at 3:58 and if she's sending a text when it does, she's clearly not in the trunk or in distress. Maybe it was a food run, a weed run, a quick jog to parents (all of which would be an expected round trip to Shops), or maybe she's being dropped at parents. But it's hard to imagine enough time from text to struggle to trunk before the car is exiting, and the content of the text hints that she's still okay when it was sent.