For some reason, I think the light bulb came on in RH's head when he had the conversation with LH around 4:00. Possibly, the reason he told his friends he'd be a little late for the movie was because he planned to stay at work until 5:00 due to his late arrival that morning. Then, something triggered his memory around 4:00--he ended up leaving very soon after he spoke with his wife. Maybe he said something to her then that gave her an idea that something bad had happened, and when she got to the day care and didn't find Cooper, she said the odd things she said. RH could have known when he got to his car that his baby was dead, but didn't want to have this whole scene unfold at his workplace. He might have decided to go home but wasn't able to cope anymore so he pulled over. Maybe LH wasn't acting like someone would act the first time they heard they're child was dead because she actually already knew after that phone call and had already endured the initial shock reaction.
Just one of many theories that roll around in my head--I still don't know how someone could accidentally leave their child in a car, but I am looking to grab something to help keep some faith in humanity...
I somewhat agree with your theory and believe something Leanna said around 4:00 triggered his memory. Everything points to Ross being lazy...too lazy to take his son to daycare. I believe he had left him in the car previously and Leanna knew it.
JMO