Perhaps she went to the gas station to fill up a gas can and it spilled inside the vehicle.
I started thinking along the lines that maybe the fire wasn't supposed to happen at all. It does not make sense to draw attention to the yard when there seems to be so much possible evidence in the yard.
EB claims she checked on Zahra around 2:30 am. The fire was called in around 5:20 am.
The dogs hit on both vehicles having decomp present.
Here is a theory. Maybe EB put Zahra in one vehicle and decided that the body was too visible by those who might pass her on the road. She moves the body into the Tahoe. The Tahoe is a work vehicle, so it's likely to have full cans of gasoline in it. In the process of putting the body in the Tahoe, a full can of gasoline spills in the Tahoe and flows out to the ground.
She disposes of Zahra's body between the hours of 2:30 am and 5:00 am. She returns to the house and parks the Tahoe a few feet away from where it was originally parked. She tosses a cigarette as she gets out of the vehicle, goes into the house. The dogs bark and wake up AB, not because of a fire but because EB is coming in the back door. The tossed cigarette smolders in the mulch and eventually ignites the gasoline.
I'm almost betting that all of this has to do with money. The plan was to extort money from the boss without getting police involved. EB (or both) planned to play the guilt card on the boss to get him to give them money to get Zahra back, after all, Zahra was missing because the boss had angered someone....his daughter is safe and Zahra is missing all because of him. Don't get the police involved, just give us money to get Zahra back.
The fire made EB (or both) have to rework the plan. It is the only theory that I can come up with where AB might not have played a part in it. Perhaps he was woken up by the dogs barking at EB coming in the door rather than by the fire in the back yard.
Just another possible theory to throw out there.