I can understand that. However I must admit that I wonder if the frustration I'm feeling over "the motive" discussion is at all similar to what prosecutors face when deciding whether or not to try a case these days.
We live in a time of advanced technology -- DNA, ballistics, etc... Cases without any of that are extremely difficult to try because people want "The Smoking Gun," or in absence of that, they want a clear motive.
This will be a circumstantial case (unless direct evidence comes to light in the future). It is also a felony murder case -- which means that he may have not meant to KILL the boy, but he was acting in such an irresponsible (aka neglectful/abusive) fashion that would have been prosecuted standing alone with or without the death of the boy (if discovered).
First the case and the evidence -- WHY must we require that smoking gun when every bit of circumstantial evidence when ADDED TOGETHER forms a huge, neon arrow pointing to NOT an "accident"?
Motive? There are famous cases with NO clear motive -- it was up to the prosecutor to try to figure it out & explain to the jury.
I imagine in THIS case, a good prosecutor could open and close with something similar to this:
Ladies & gentleman of the jury, I have no clear motive to present to you. There is no smoking gun. This case requires you to put the pieces of a puzzle together. One piece on its own means nothing -- it's the entire picture, when all the pieces are in place, that proves guilt beyond a reasonable doubt. When the defendant had JUST SEARCHED hot car deaths in the days prior, you would think it would be fresh on his mind. He forgot his child after Chick-Fil-A, within minutes -- less than 5 to be exact. He didn't remember in the morning. He didn't notice him when he went to his car in the middle of the day. He didn't remember in the afternoon. He didn't remember until he was on the freeway after work. He didn't notice a smell that was noted by officers on the scene. He claimed his child was choking. One piece by itself... horrible, unfortunate accident. All together? NOT an accident. WHY was the child left in the car? We don't know. But he was left there -- as we've established, it's doubtful he was truly "forgotten" -- and he died. Dad may not have wanted him to die, but he is dead as a result of negligence. Gross negligence/abuse that results in death = felony murder, beyond a reasonable doubt.
They have a case, IMO