I thought about this case again recently. I cannot understand why, living in the desert, he didn't take a shovel , pack her bags and go and bury both the body and her stuff in the middle of the desert.
Sure, he'd have been a suspect in her disappearance but prosecutions without a body are extremely rare. Just tell the police she had problems -hint at lots of drug taking - stole some stuff and left.
Burning her - unsuccessfully - in your own car and going and buying the accelerant late at night using your store card, fleeing, getting caught because your wife wanted a manicure, that is a remedial level intellect at work.