I keep thinking that RH and LH discussed freeing Cooper from this difficult world together. I think that RH brought up the idea of leaving Cooper in the car and maybe told LH that t would be simple, peaceful, painless, etc. and so LH went on the websites after RH to confirm what RH was telling her. I think that they agreed on the plan together and that is what led to the comments regarding dreading what Cooper would look like and that he had his eyes closed and then LH asking about RH saying too much. I think all of those post-death conversations were follow-ups to their pre-death conversations. While I know that two parents agreeing ahead to murder their child seems almost impossible, I do think that is exactly what happened here.
In regards to forgetting your child in the vehicle I do admit that I forgot my wee one once right after she was born. I don't remember now how long we had been in the truck, but I do know that we were only a block or so away from home when it happened so we must have been on our way home. I had my three children in the truck (8,almost 6, and newborn) and my older two were in the seats next to, and behind my newborn (3 rows). We pulled up right in front of Subway, got out of the truck together, went into Subway, ordered, and started to sit down when I looked out of the store window at my truck and realized that I had left my baby in there. Luckily it was during the late Winter months and therefore not hot, plus she was sleeping the entire time so she was not crying or upset by my carelessness. I was not a new mom, and I was not a young mom, but I was distracted by my older children and I was likely exhausted and I was used to having just two kids to get in and out of the car. My point is that it does happen to good parents; however, in this case I do not think it was an accident at all.