Don't most children this age know how to unbuckle their seats and how to open a car door, even unlock it? I would imagine, if the child was asleep when dad got to work and woke up either from door being closed, or later when the nap was finished and the car was hot, the child would try to unbuckle and get out at that point. I am guessing he was already passed before being put in the car, and maybe livor mortis (am I spelling it right?) showed he wasn't in that position until some time after death. Whatever the cause, I hope it was quick with no suffering!
No, and that's really why it's so dangerous. Most car seats for kids that age can't be unlatched by a child. You stick them in there, and they're tied down and can't move. Which I guess, is generally safer than having kids keep getting out of their car seats on the highway - which is what would happen if kids were able to unlatch themselves.