"Question is though, would this allow you to walk half way off screen?"
Yes! Try with DDCheat, and check the Y position, there are some Y values to do not belong to any screen. Try to move between screens very very slowly with DDCheat and you'll understand what I mean.
On another topic:
I find the order of objects in the array quite interesting. It lets you have some basic understanding on what earlier versions of the game looked like.
