Just a few of my thoughts on this...been reading to catch up. That text JRH sent to his wife that day, "don't forget to pick up my buddy" could be a code. The use of the word "forget" when he was the one who forgot cannot be a coincidence with everything else. I believe that Ross didn't want to be the one to walk in to the daycare to pick him up when he hadn't dropped him off because he knows he isn't a very good actor. She walks in knowing that he left Cooper in the car and her brain outsmarts her and she blurts it out.
If they planned this together, which I think they did, they probably didn't imagine he would be arrested and they would be together at the funeral. She hadn't planned doing that alone and needed his help on the phone to get through it. Although he did search the internet for how to get by in prison, I bet they also searched to see how many parents get arrested for this crime and felt the odds were in their favor.
I believe that he did himself in by admitting that he finally noticed Cooper in his carseat while making a right-hand turn. This is something he had done while driving to work and while backing up. The real horror will come to the jury when they witness the 30 seconds this man sat in that car with Cooper before exiting the vehicle and walking away. Thirty seconds doesn't seem like a lot but when they actually watch that much time go by, and realize that the child is awake and alert inside with him.....that will seal his fate, imo.