A force applied through a solid has a greater effect than the same force applied through a gas, especially a gas in a generally open system like the air is. You seem to forget your force diagrams. If an object is perfectly vertical, it'll balance and stay vertical. If it is even slightly off vertical and without a stable base, the force would still be applied vertically, but the force as it would appear from the object's perspective would be off vertical, and thus have a vertical and horizontal component, causing it to tip. (this is something like trying to balance a stick). For the board over the lip of a flat roof, its a matter of leverage.
_________ ___________ ___________
^ ^ ^ ^ ^ ^
stable near leverage point unstable
The stable diagram will not fall since the force is applied throughout the object (completely on the roof). The middle diagram will not fall as long as there is at least equal mass on both the force side and the non-force side of the object (object partially hanging off the roof). The unstable diagram will fall since only a small portion of the object is getting the force applied.
In your car example, if you hit the wall off center, and say this wall were on a swivel and could rotate, it would spin. Basically the same situation as above, just horizontal instead of vertical.
Your diagrams are perfect, except you forget to draw a downward force. In a stable force, the force acting down is equal to the force pushing up. If there is only one force applied, as in your model, the model will continue to move upwards. Even in the unstable situation, with just one force, it will continue to move upwards. However, and that is where you think right, the object will start to fall over in an unstable situation. That is because the force acting down is greater than the force acting up. As I said, if one particular force is greater than the other, the object will move. You can hold any object in your hand. Once you move it in any direction the force in that direction is greater, so you move.
The bricks also don't happen to fly in the opposite direction if the car hits it in one direction, or does it? That is the lack of force acting in the other direction. The only reason why the bricks stay behind and the car moves on, is because the bricks are on the road and not on the car. If the bricks are on the car, you could hold them onto there.
I want you to take a piece of paper, this is a very easy experiment, to show you exactly what I just said. Now hold the paper in your open hand nice a balanced. It does not go anywhere, right? Now move the piece of paper from the middle of the hand to where a point it will start to fall. You now have your reference point. Now take the paper again and hold in onto your hand with your thumb at the point it would otherwise fall. Now stand up. Put your hand vertical, so the piece of paper is vertical too. Like in the following picture. Now start to spin around. You can let go of your thumb and you will see the piece of paper will stick to your hand. Now at some point the paper will fly off your hand as if you start to play around with it, this is due to the fact the air that surrounds it does not move with you and it will act as a force on its own. You would have to go a whole lot faster, do you want to overcome that force. I cannot go any faster so I cannot maintain the paper on my hand at all times. The earth, with all the objects and air on top of it, is constantly accelerating upwards and does not have that same problem. Therefore, even if the object is off balanced, with only an upward force the objects will stick to it. Only a free object will appear to fall, because the earth is moving upwards.
You are right, it is not what we observe and that my friend is gravity.