I don't think Ross forgot him on the drive, I think he forgot him while at work. I think he meant to leave Cooper in the car, but just for a minute or two while he made an appearance at work. He was late for work, again, and he wanted to "check in." I think he intended to return to the car and bring Cooper to daycare, but forgot.
His ex-wife claims she is the one who searched for hot-car deaths. I think she did this because Ross had used the car as a short-term babysitter in the past, and it scared his ex-wife. She wanted to prove to him how dangerous it was. When she found out Ross didn't drop off Cooper, I think she knew what happened - he forgot him in the car.
My gut tells me he "forgot" Cooper, but because he had done this in the past, it was a neglectant habit of his. He put his son in danger. He took a terrible risk with his child's life and that is his responsibility.
I'm tempted to lean toward a deliberate act of murder because of his immature attitude toward marriage, but my stronger hunch is that he forgot Cooper in the car while he was in the office, not when he was in the car. He didn't intend to kill Cooper, but that's what happened from his taking a known risk. He remembered during the day, but it was too late. He knew he had a track record of leaving Cooper (my speculation), and he covered his actions with the phony discovery on the way to the movies.
JMOpinion.
I really think that is the most plausible scenario, in part because it explains a lot of what otherwise doesn’t make sense.
He began texting with a woman on Whisper at 8:55.19, while still in CF, or as he was leaving CF, or when he first got into his car. I’m assuming he was supposed to be at work at 9AM, which means he was already running late/cutting it close, even without the time it would take to drop Cooper off at daycare.
He didn’t forget in 30 seconds or 1 minute or 2 that Cooper was in the car. As you said, he took Cooper to work so he wouldn’t be late, intending to clock in or be seen, and go right back out to drive Cooper to daycare.
He wasn’t paying attention to people in the parking lot because he had no reason to care. He was checking his phone because his mind was on the Whisper woman (WW)—not Cooper, and not on work.
The timing of the next texts supports this scenario as well. No texts after 8:55:19 until IIRC, around 9:12. The amount of time it would have taken to get inside the building, and do whatever first things. WW sent a text, he sent a text, the texts went on until around 9:23 or so, again IIRC. Long enough to have thoughts about Cooper replaced by whatever he was thinking while texting with WW.
It makes no sense that he’d deliberately bring Cooper to work (or anywhere else, IMO) and “forget him” if it was standard daycare policy to follow up with the parents of unexcused children who were expected to be there. He’d have to be concerned that the daycare would contact LeAnne if they couldn’t reach him.
One problem for Ross is that telling the truth about what happened wouldn’t help him much, if at all, if this is what happened. He’d have to confess to LE and to LeAnne that he intentionally left Cooper in the car for no better reason that he went into work and forgot about him.
If this is what happened, then I don’t believe it’s possible he went to his car at lunchtime without realizing he’d left Cooper there, in that hot car, for hours. IMO he’d have had to realize Cooper was either dead or in critical condition. Buying and then throwing the lightbulbs in his car becomes a likely way for him to have looked at Cooper briefly, to see if he was already dead, without having to decide what to do next. He didn't tell LE about his lunch time trip to the car because neither explanation would help him--he returned to the car without realizing even then he'd left Cooper there, or that he saw Cooper and knew he was dead but did nothing.
It will be interesting (though not conclusive) to see if his text records evidence indicate whether or not he stopped texting after lunch, or if he returned from lunch and kept right on texting.
Knowing that Cooper was already dead makes sense of his call to friends that he’d be late to the movie although he had more than ample time to not only be on time but to be early. The screeching into a parking lot—staged, but also a product of adrenaline, same as his hostility to LE when they arrived.
I can believe he did cry and get very emotional at the scene before LE arrived. He didn’t do CPR because he knew Cooper had died hours before, but because I don’t believe he intended to kill his son, I can believe he felt genuine grief (in his own fashion), and this was his first opportunity to express it. Briefly, because since he had known for hours Cooper was dead he had had hours to think about what he was going to tell LE, and they were on their way.
His—“I didn’t want to imagine how he’d look “ was his way of telling LeAnne what had happened, and that he felt “remorse.” Her lack of affect then and afterwards was perhaps because she blamed herself for allowing Ross to drive Cooper again, after he’d repeatedly demonstrated negligence.
So… not malice murder, but if this scenario is accurate, I’d be on board with throwing the book at him.
ETA- even if this isn't what happened, he'd be better off IMO he told this story, rather than to insult the intelligence and sentiments of an unsympathetic jury by claiming his brain got scrambled and his son died a horrible death because he had a sexual addiction that led him to sext with 6 women while Cooper died, in severe pain, alone.
This was how the guy operated. 
