A few things I'm wondering about. Are the oil tanks involved in this case big enough for an adult to fit through the opening? Did he intend to put SW in one as well? Was she too big, being an adult, for him to get her into the tank? Where was the oil destined for? Are the tanks ever drained otherwise? When oil is removed from the tanks, is it for processing? Sale? Transfer? Would decomposition byproducts become noticable? Would the oil restrict or aid decomposition? Are they going to find internet searches on these types of questions? Were the oil tanks something he thought of before or after the murders?
In spite of things that have rubbed others the wrong way (I get it), I feel like N bit down like a pit bull and wouldn't let go when she sensed something was wrong. Had she not been so persistent and involved LE so quickly, it's possible that CW would have had more staging time, making this case that much harder for authorities to solve and prosecute.
Edited for spelling.