I hope this didn't happen, but if her son fell asleep and didn't awake when they became stuck, she could have picked him up. In the dark in her dazed state, she could have then carried the sleeping boy the wrong way -toward deeper water instead of dry land.
That thought is chilling.
But I think the cold water would have awoken the boy. I have to say I am really really bad with directions and so I can understand her confusion, however she must have made more calls to the Fiance or her stepfather if she was lost. Why would she go through all the trouble to stop at a random home and call her grandparents??? - Why not call the fiance, see where he is and if he can meet them because they are lost, call the stepfather and do the same? Did she do this? Are there voicemails to his dead phone?
If she had money why not stop and get change for a payphone? Why leave the corndog, did she see something or someone that made them leave?
Were there any other items int he car, junk food that they bought?
Did she think she was arriving on a dirt road and would find another house to make a call and then found herself int he water - was she sleepy, sick?
Did she stop at another home that we don't know about? Was she taken and then someone else tried to submerge her car but changed their mind?
The odds of this being a crime of opportunity are low.
It has to be an accident or a planned crime. IMHO
