Something I haven't seen brought up much is the financial aspect of all of this.
Erin gets pregnant by CL.
Erin knows CL is planning on getting out and moving to Alaska, far far away from her.
Erin tells him that she may leave her husband and he had better be prepared to pay child support for the next 18 years.
CL realizes that she could soak him for a ton of money, if the baby is in fact his. $800 mo. (+-)
CL starts to plot and plan on how to rid himself of this problem. (Was his wife included in this planning ? ) Unknown.
CL finds a mine shaft, takes pictures of it on his phone, lures Erica to go with him for a day of target shooting.
Finding those pictures might have been why law enforcement continued to search so hard at joshua tree.
We all know the rest of the story. Erin may have died due to the oldest motivation known to man--GREED
Interesting theory steelman. I will only add that I do believe Chris and Nicole were in it together. Perhaps this financial aspect (rather than plain old jealousy) was the reason why Nicole wanted Erin dead. Nicole probably felt that it was unfair that her family would have to suffer financially for Chris and Erin's mistake. In any case, I agree with you that it was mostly about the money.