I think you are misunderstanding the information you have found.

First. Football concussion research looks at the G forces acting on the skull at the surface of the skin, not the forces acting on the surface of the helmet.

The fall into a chair causing 10 G's sounds very high. If the chair is rigid, and one lands with the back arched (straight vertebral column), and there is minimal absorption through the back side, the G forces could be that high as they are transmitted up the spine. But, a cushioned chair and/or a normally curved back will greatly reduce these forces.

The tennis ball number is also misleading. Is this 3 G's acting on the tennis ball or 3 G's acting on the skull. The fact that a tennis ball distorts means the G force is reduced.

*admin edit*

Regarding sneezing, there is a whiplash effect causing G forces plus a pneumatic pressure in the sinuses.
Here is a good but complex explanation of calculating G forces. http://www.decodedscience.com/mythbu...alling/26999/2