Negligence should be a given. Even criminal negligence, considering the outcome. You place the child in the car, you are responsible for his safety to the best of your ability. This precludes another car hitting you, but should include things like the child getting free due to improper buckling, or being left behind to die, for instance. It was within RH's power to safely deliver his child to daycare, but he did not...for whatever reason. To me, that is enough for criminal negligence, though the law may not agree. I am probably wishing a bit.
Now to show intent is going to be much more difficult, I think. For me, the prosecutor may have a harder time showing he set this all up to the satisfaction of a jury, than the defense will be in turning into "something that could happen to anyone", unless Mom speaks up or the computers uncover more intent-leaning evidence.
As of what we know right now, the defense has its two worst challenges in his sexting relationships/activity, and the google searches. But if jurors don't want to believe a parent could plan a horrible death of their baby in such a gruesome manner, I could see them bypassing those two issues and not being able to conclude intent. All jmo