Something totally different has been going through my head today, and I've finally decide it might have some merit so I should share it.
I've been wondering if they were trying to find ways to save money, and decided to see if they could cut daycare costs if they only brought CH in part time instead of full time. If he brought him to work with him and left him in the car, then she picked him up a couple hours later, she could drop him off at daycare on days she had appointments or take him home on days she didn't. If he left him there at 9:30 that day, maybe she was supposed to pick him up by 11:00 just to see how he'd handle a short period of time out there alone.
She may have gotten busier than expected and forgot, or maybe they got the days mixed up and she thought they were going to try on a different day. If she was with clients, it would make sense to turn her phone off, and she may not have gotten the message about picking him up until just before they talked at 4:04. He may have really bought the light bulbs innocently and been shocked to see CH in there dead. Their odd behavior that day and in court could be that they were in shock, feeling guilty and had no idea how to handle it.
I just have a hard time seeing such seemingly loving parents change so suddenly and killing the child they had loved in cold blood. It would still make him guilty, but because he left him there on purpose even if he hadn't meant to kill him. Hope I made this clear enough, I haven't figured out all of the details yet.