IMO:
I really, really, really (unfortunately) do not think this was an accident at all. Like, at all. The fire was the cover up for whatever horrible crime happened to her beforehand. No rolling drug labs, no huffing or drug use accident, no recalled gas tank explosion, no rear-end collision gone bad, no hit and run, and she didn't run off the road - IMO it was nothing like that at all. The person/people thought fire would destroy all DNA evidence. I'm just stating my opinion, and I wish it were an accident, because this is pure evil. I'm not saying anyone else is wrong, either, because I wasn't there, I am not clairvoyant, and I definitely don't know everything (ha!).
This perpetrator probably knew her. I don't think the person acted alone (at least when it came to the fire) because there had to be a getaway, and they obviously had to have their supplies to start the fire. They had to get rid of the supplies (empty containers, etc.) because I have yet to hear that any of this was found at the scene. I think they certainly thought she was dead before they started the fire. Since the phone wasn't burned, I think they took it before they lit the fire, maybe even as an "oh crap - grab her phone" last-minute thing, removed the battery, threw it down and thought that was sufficient as far as the phone goes. Seems they didn't think very hard about the phone. They obviously do not watch a lot of true crime TV. Anyone who followed the Casey Anthony trial (and others), as well as watches the ID Channel, religiously, knows all about cell phones and what can be found out. I think the rear-end that looks damaged is most likely warping and melting from the fire, etc., if it wasn't there before. I think an accelerant was used due to the extent of the damage, the height of the fire (look at that one tree), etc. (they just won't say for sure yet). Maybe they are waiting to determine the type of accelerant first.
I think this was either a jealous person/people who assaulted her (probably somewhere else) OR
a former boyfriend or hook up who was abusive and thought if he couldn't have her, no one could and this was preplanned OR
an abusive guy who she went to talk to, they had an argument, he hit her on the head in the moment,
thought she was dead and tried to burn the evidence OR
she was raped and they tried to burn the evidence, including in her mouth and nose (IF something was really poured in them).
Those are the possibilities I am going with, and I seriously hope this is solved sooner rather than later.