The reason I even brought it up was because I knew a woman who did commit suicide by fire. It was a lesson I didn't want to learn.
Suicide is tragic enough, but to do so by fire - that strikes me (and I think most people) as a horrible, terrible, awful way to die. I am really sad to hear that someone you knew was hurting so badly that she chose to end her life like that. :-(
It is also a relatively rare way to do so, in general but particularly among females in the U.S. The CDC as well as the American Foundation for Suicide Prevention have data that backs this up. The most common methods for both genders combined are firearms, poison (overdose), and suffocation (including hanging). The remaining "Other" is just under 8% of all methods - and that is for men and women alike. I can split this further, but the point is it's not impossible, but not a popular means at all - so much so that when we hear about it, it's even more shocking.
And now I'm going to walk through the suicide scenario as you're presenting it and point out where parts of it are either implausible or require a stretch of the imagination.
The parking brake was on and the car was not crashed indicating she parked there on purpose. She purchased more gasoline than normal which could be she thought the vehicle would burn faster. The blunt force trauma to her head could have come from the concussion of the explosive force of lighting the liquid accelerant. The phone could have been in her hand and blown out the window when she lit the fire. It could have broken apart when it landed or she could have thrown it prior to starting the fire. She could have gotten out of the car in an attempt to save herself or because the pain was so overwhelming.
All of this is indeed possible. The facts of the case as we know them at this point DO allow for this scenario. But your description supposes a number of things that, each on its own, could well have happened, but what is the reasonable likelihood of ALL of them happening versus foul play?
If we accept that Jessica chose to commit suicide this way, then we assume that okay, she filled her gas tank more than usual around 6:30pm, possibly thinking her car would burn faster that way. But some point AFTER that yet BEFORE 8:15pm, she speaks with her mother on the phone for about 20 minutes. (We don't know what that conversation was about, but we have not heard any indication that she was upset or saying goodbye or anything of the sort to indicate she was off to kill herself - but she's already bought that extra gas at this point, right?)
Furthermore: at some point (and not AT the gas station, at least based on CCTV footage), Jessica then puts a flammable liquid on herself and the interior of her car. This could have happened before or after she drove herself to that location and purposely parked the car there. Then, she ignites the fire - a match? a lighter? - and there is an explosive force that occurs as the fire catches. It is so strong that Jessica hits her head and even if we assume she took her seatbelt off - this could have happened... on the ceiling of her car, or on the side or window of her car, or on the steering wheel. That's all I can think of.
Lighting a fire with one hand while holding your phone with another is... tricky, but not impossible. Lighting a fire with one hand, holding your phone in the other, and immediately thereafter simultaneously hitting your head so hard to produce a visible injury as your phone then flies out the window only to come apart with battery separate from the body of the phone is... dang.
We also have to assume her window was either open or that it was blown out by the fire at the same time for the phone to fly out without such severe damage to the phone itself (it was identifiable and recoverable, with evidence obtained from it).
Her phone can't well fly out of the car, though, if her door is closed and her window is up.
She could have, yes, as you said - thrown it out of the window prior to lighting the fire. This seems more physically likely. Depending on what kind of phone she had, separating the battery from the phone itself could have been easily managed in a throw or more complex and required her to disassemble it before throwing.
Lastly, she exits the car on her own volition, suddenly trying to save herself or perhaps because the pain is more than she'd bargained for and she wants to minimize it. Firefighters arrive not long thereafter.
First post, I've been meaning to jump in forever, now's the time. 
