I initially thought that LeAnna was involved. After her testimony, I found her genuine and sincere regarding Cooper--I don't think she was involved in any way. A lot of people claim because she said "he must've left him in the car" is the indicator of her guilt. However, she had JUST talked on the phone with him an hour prior and had texted with him back and forth during the day---the discussion of which was for her to pick Cooper up at daycare. So basically: Ross told her that Cooper was at daycare and that she was supposed to pick Cooper up. Then when she gets there, the daycare workers say "Ross never dropped him off." What is the only logical explanation then? He must be in the car.
Additionally, if she had known of the plan, then she would've known of Ross's desire for her to stay at the daycare. Either she didn't know anything about Ross's plan, or she forgot her lines in the play because she didn't stay at the daycare and instead went to the Treehouse, looked for his car, and then proceeded to go in and out of the building trying to charge and use her phone.
Her behavior at the police station is also IMO, understandable--though, I admit, strange. It seems as though in that family there were two children, and one adult. The adult, LeAnna seems to have had her "mothering mode" kicked in, just as a parent who loses one child will not fail to comfort the other---even at her own expense for mourning the loss of the first child.
When she referred to hot car deaths as "we talk about it all the time." I betcha the "all the time" had been in the previous two weeks as Ross was hatching his plan. Probably sizing her up, preparing her for his forgetfulness, and disgustingly so, reveling in his fascination and excitement for what he was about to do. I would be willing to bet he brought that conversation up many times in the weeks prior to Cooper's murder. It is clear "hot car deaths" was a legitimate fear of LeAnna's because she followed those stories and sent those stories out to people (including Ross) in her email in the years proceeding Cooper's death. As one person on here stated, I think this manner of killing Cooper by Ross was a big "screw you" to LeAnna, I'm going to kick you where it hurts--so you'll hate me and want to divorce me. Bet he was surprised when he fished for her response to the situation in the police station, and she said she would "never leave him" and "doesn't hate him."
She nearly collapsed during testimony when she (obviously) thought the prosecutor's line of questioning was going to lead to her seeing pictures of Cooper deceased. I'm confident she loved Cooper, still does, and had nothing to do with his death. However, I think her top priority was Ross and maintaining her marriage because she inquired about having more children and mentioned it at Cooper's funeral. I think she knew this was the only thing keeping Ross in the marriage and felt with Cooper's death, he may leave her once he got his bearings back after "the accident." All that is unfortunate, because had Cooper been her top priority, she may have evaluated the evidence differently and have come to the same conclusion the majority of us have: Ross systematically planned and executed his son in a heinous, tortuous way in order to leave his marriage with no strings attached.