Katydid----I'm going to go out on a limb here and answer your question, but it's going to make some people in here very angry at me. It won't be the first time I've made a bunch of folks mad, probably won't be the last either.
The answer: Diana turned the burners on
Hold it ! Hold it ! Hold it !..............here's why.
From the very beginning, I thought the burners being turned on in order to set the house on fire was rather amateurish and silly. Setting an entire house on fire in that manner would only work under optimum conditions. My theory is that Diana was instructed to do that by Jorge. I'm guessing that the burner under the skillet was probably set on LOW temperature, thus it taking it so long to heat up before anybody noticed it was on and smoking. Why turn the other burners of the stove on if there wasn't anything sitting on top of them ? If I recall, the parents said all the burners were on, and a skillet full of oil was sitting on top of ONE of them.
Being 9 years old, and having Jorge as a mentor, she may've not really understood completely why he was telling her to do this or what significance it had. She obviously was in no position to make "grown-up" decisions. Hell, I'm barely capable of making "grown-up" decisions, and I'm in my late forties. That aside, Jorge may have instructed her what she needed to do before exiting the house. Knowing now that there was some sort of communication going on early that morning between them, I'm led to believe that Diana was a willing cohort in this scheme, although she had no idea exactly what Jorge had in store for her.
She trusted this guy, and she did what he told her to do. It was part of their "secret".
I think there's a good possibility Jorge pulled up outside in a waiting car, and Diana came out to meet him. He likely never set foot inside the trailer that morning. Turning on the burners of the stove in the manner it was done seems rather "childish"...................get it ? Maybe in the back of her mind, Diana did realize that turning the burners on high would be dangerous, so she didn't do it, unbeknownst to Jorge.
That oil could have set there and been heated for hours if it was on low or medium-low setting. It would have gotten hot, but not hot enough to cause a flash fire, which I believe was the intended result.