Am right with you SteveP -- your post's well-expressed and a clear-headed assessment throughout -- but at my red "x" our thoughts diverge: mine go to the person dealing with the sea of problems by withdrawing forever from them, opposing them in that manner and thus ending them....a sad thought, yes, the opposition that would result in letting all of life go, albeit a life which must have been brutally hard for a variety of reasons, and which, as you well note, from which she may have been unable to escape. I think the victim must have succumbed to sadness and defeat.
The extra gas purchase, said to be thrice her usual buy, argues, I submit, not necessarily to one about to take a longer journey than usual -- at least on earthly roadways.
True, LE keeps telling us homicide, homicide, homicide -- but I don't put a great deal of trust in this particular crew. (I lived in OK for 15 years and think I here recognize a sort of negative commonality with LE I've seen.) Do they know more than me? Hell, I hope so! But I think with reputations at stake it can be human nature to cling to what may in the long run turn out to be mistakes made then multiplied from the case's beginning. Doesn't make these folks bad people, if they are proven wrong. No one's perfect.
One version of Occam's Razor suggests that entities must not be multiplied unnecessarily, at least in order to arrive at the simplest and thus most probable solution. But I sense that sort of convolution happening here. I may not be right -- in fact, in spite of my linking above several cases in which car fires were in actuality successful suicide attempts, my solution is not an odd's on favorite. I do, however, think it must be deeply considered by anyone taking a serious look at the psychology of the individual who perished so painfully in this case, as if, perhaps, punishing herself for a life of youthful missteps -- missteps which led reponders to a tragic scene at a secluded spot.
Requiescat in pace Jessica. May your soul be always at peace and your atoms be mingled with the shining stars above.