Can you re-enter into the fair grounds after you've already been inside the fair?

Yes they stamp you for the color of the day

Yes, with a hand stamp b4 u exit

Yes, as long as you ask for a hand stamp from the gate attendants before leaving the Fair.

To put your prizes in the car or yoir left over of food. There's a boat load places

You can get a re entry stamp at the entrance/exit area