Iacisrmma, I don’t need to dispute any of the facts you presented to assert that it is still far more likely that this was a case of forgotten baby syndrome than a case of a negligent (to intentionally leave a child under any circumstance) and ignorant (to not know the car gets dangerously hot) or worse, callous and unloving mother who doesn’t care if her child dies (not that you claimed this).
These are so unlikely, to the point of being preposterous, but my explanation (that she met her other children in the store) or Joseph’s (that she thought the baby was with the other children), or other possibilities are far more likely. The halachah us that we must assume the mist favorable, and it’s not even a stretch here under the circumstances.
Your disclaimer is very nice, but doesn’t explain why you wrote, “I really would like to be Hevei dan es kol haadam l’kaf zchus. I am sure the mother is agonizing over this. But you have a 13 year old and 7 year old with you in the store and none of you realize the baby is not with you?”, and why you keep focusing on the worst (and least likely) explanation, even if you’re uncertain. If you would like to be dan l’kaf z’chus (as if it’s optional…) the opportunity is right in front of you.