Meanwhile, I don't know what to tell you. You are so positive that you know it all, it is difficult to explain anything. You won't believe me, but maybe you'll believe a video. But look at the video, and see these riders hitting a really slippery part of the road and going down. Your box where the two forces (vertical and horizontal) form the hypotenuse of a triangle are correct, but that force is translated to the body only if there is an abrupt deceleration. If there is no abrupt deceleration, then the riders simply continue to slide, as is happening in this photo and part of the video. This shows very clearly that only the vertical component of the fall is important here.



It's like the MIR space station accident, where a robotic ship rammed the MIR. They were going over 17,000 mph, but none of that was translated to the ships, as they were going in the same direction. Only the closing velocity was translated, and because the supply robotic ship was about two tons, even at a slow closing rate, it punched a hole in the hull of the MIR.

http://www.encyclopedia.com/doc/1P2-722152.html

As my Parachute Jump School (US Army, Fort Benning, GA) instructor told me so long ago (1967), "It ain't the fall that kills ya, it's the sudden stop."